Network Working Group J. Moy
Request for Comments: 2178 Cascade Communications Corp.
Obsoletes: 1583 July 1997
Category: Standards Track
OSPF Version 2
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo documents version 2 of the OSPF protocol. OSPF is a link-
state routing protocol. It is designed to be run internal to a
single Autonomous System. Each OSPF router maintains an identical
database describing the Autonomous System's topology. From this
database, a routing table is calculated by constructing a shortest-
path tree.
OSPF recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic. OSPF provides
support for equal-cost multipath. An area routing capability is
provided, enabling an additional level of routing protection and a
reduction in routing protocol traffic. In addition, all OSPF routing
protocol exchanges are authenticated.
The differences between this memo and RFC 1583 are explained in
Appendix G. All differences are backward-compatible in nature.
Implementations of this memo and of RFC 1583 will interoperate.
Please send comments to ospf@gated.cornell.edu.
Table of Contents
1 Introduction ........................................... 51.1 Protocol Overview ...................................... 51.2 Definitions of commonly used terms ..................... 61.3 Brief history of link-state routing technology ........ 91.4 Organization of this document ......................... 101.5 Acknowledgments ....................................... 11
2 The link-state database: organization and calculations 11
2.1 Representation of routers and networks ................ 11Moy Standards Track [Page 1]

RFC 2178 OSPF Version 2 July 1997G.4 A modification to the flooding algorithm ............. 206G.5 Introduction of the MinLSArrival constant ............ 206
G.6 Optionally advertising point-to-point links as subnets 207
G.7 Advertising same external route from multiple areas .. 207
G.8 Retransmission of initial Database Description packets 209
G.9 Detecting interface MTU mismatches ................... 209G.10 Deleting the TOS routing option ...................... 209
Security Considerations .............................. 210
Author's Address ..................................... 2111. Introduction
This document is a specification of the Open Shortest Path First
(OSPF) TCP/IP internet routing protocol. OSPF is classified as an
Interior Gateway Protocol (IGP). This means that it distributes
routing information between routers belonging to a single Autonomous
System. The OSPF protocol is based on link-state or SPF technology.
This is a departure from the Bellman-Ford base used by traditional
TCP/IP internet routing protocols.
The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force. It has been designed expressly for
the TCP/IP internet environment, including explicit support for CIDR
and the tagging of externally-derived routing information. OSPF also
provides for the authentication of routing updates, and utilizes IP
multicast when sending/receiving the updates. In addition, much work
has been done to produce a protocol that responds quickly to topology
changes, yet involves small amounts of routing protocol traffic.
1.1. Protocol overview
OSPF routes IP packets based solely on the destination IP address
found in the IP packet header. IP packets are routed "as is" -- they
are not encapsulated in any further protocol headers as they transit
the Autonomous System. OSPF is a dynamic routing protocol. It
quickly detects topological changes in the AS (such as router
interface failures) and calculates new loop-free routes after a
period of convergence. This period of convergence is short and
involves a minimum of routing traffic.
In a link-state routing protocol, each router maintains a database
describing the Autonomous System's topology. This database is
referred to as the link-state database. Each participating router has
an identical database. Each individual piece of this database is a
particular router's local state (e.g., the router's usable interfaces
and reachable neighbors). The router distributes its local state
throughout the Autonomous System by flooding.
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RFC 2178 OSPF Version 2 July 1997
All routers run the exact same algorithm, in parallel. From the
link-state database, each router constructs a tree of shortest paths
with itself as root. This shortest-path tree gives the route to each
destination in the Autonomous System. Externally derived routing
information appears on the tree as leaves.
When several equal-cost routes to a destination exist, traffic is
distributed equally among them. The cost of a route is described by
a single dimensionless metric.
OSPF allows sets of networks to be grouped together. Such a grouping
is called an area. The topology of an area is hidden from the rest
of the Autonomous System. This information hiding enables a
significant reduction in routing traffic. Also, routing within the
area is determined only by the area's own topology, lending the area
protection from bad routing data. An area is a generalization of an
IP subnetted network.
OSPF enables the flexible configuration of IP subnets. Each route
distributed by OSPF has a destination and mask. Two different
subnets of the same IP network number may have different sizes (i.e.,
different masks). This is commonly referred to as variable length
subnetting. A packet is routed to the best (i.e., longest or most
specific) match. Host routes are considered to be subnets whose
masks are "all ones" (0xffffffff).
All OSPF protocol exchanges are authenticated. This means that only
trusted routers can participate in the Autonomous System's routing.
A variety of authentication schemes can be used; in fact, separate
authentication schemes can be configured for each IP subnet.
Externally derived routing data (e.g., routes learned from an
Exterior Gateway Protocol such as BGP; see [Ref23]) is advertised
throughout the Autonomous System. This externally derived data is
kept separate from the OSPF protocol's link state data. Each
external route can also be tagged by the advertising router, enabling
the passing of additional information between routers on the boundary
of the Autonomous System.
1.2. Definitions of commonly used terms
This section provides definitions for terms that have a specific
meaning to the OSPF protocol and that are used throughout the text.
The reader unfamiliar with the Internet Protocol Suite is referred to
[Ref13] for an introduction to IP.
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Router
A level three Internet Protocol packet switch. Formerly called a
gateway in much of the IP literature.
Autonomous System
A group of routers exchanging routing information via a common
routing protocol. Abbreviated as AS.
Interior Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous System has
a single IGP. Separate Autonomous Systems may be running
different IGPs.
Router ID
A 32-bit number assigned to each router running the OSPF protocol.
This number uniquely identifies the router within an Autonomous
System.
Network
In this memo, an IP network/subnet/supernet. It is possible for
one physical network to be assigned multiple IP network/subnet
numbers. We consider these to be separate networks. Point-to-
point physical networks are an exception - they are considered a
single network no matter how many (if any at all) IP
network/subnet numbers are assigned to them.
Network mask
A 32-bit number indicating the range of IP addresses residing on a
single IP network/subnet/supernet. This specification displays
network masks as hexadecimal numbers. For example, the network
mask for a class C IP network is displayed as 0xffffff00. Such a
mask is often displayed elsewhere in the literature as
255.255.255.0.
Point-to-point networks
A network that joins a single pair of routers. A 56Kb serial line
is an example of a point-to-point network.
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Broadcast networks
Networks supporting many (more than two) attached routers,
together with the capability to address a single physical message
to all of the attached routers (broadcast). Neighboring routers
are discovered dynamically on these nets using OSPF's Hello
Protocol. The Hello Protocol itself takes advantage of the
broadcast capability. The OSPF protocol makes further use of
multicast capabilities, if they exist. Each pair of routers on a
broadcast network is assumed to be able to communicate directly.
An ethernet is an example of a broadcast network.
Non-broadcast networks
Networks supporting many (more than two) routers, but having no
broadcast capability. Neighboring routers are maintained on these
nets using OSPF's Hello Protocol. However, due to the lack of
broadcast capability, some configuration information may be
necessary to aid in the discovery of neighbors. On non-broadcast
networks, OSPF protocol packets that are normally multicast need
to be sent to each neighboring router, in turn. An X.25 Public
Data Network (PDN) is an example of a non-broadcast network.
OSPF runs in one of two modes over non-broadcast networks. The
first mode, called non-broadcast multi-access or NBMA, simulates
the operation of OSPF on a broadcast network. The second mode,
called Point-to-MultiPoint, treats the non-broadcast network as a
collection of point-to-point links. Non-broadcast networks are
referred to as NBMA networks or Point-to-MultiPoint networks,
depending on OSPF's mode of operation over the network.
Interface
The connection between a router and one of its attached networks.
An interface has state information associated with it, which is
obtained from the underlying lower level protocols and the routing
protocol itself. An interface to a network has associated with it
a single IP address and mask (unless the network is an unnumbered
point-to-point network). An interface is sometimes also referred
to as a link.
Neighboring routers
Two routers that have interfaces to a common network. Neighbor
relationships are maintained by, and usually dynamically
discovered by, OSPF's Hello Protocol.
Adjacency
A relationship formed between selected neighboring routers for the
purpose of exchanging routing information. Not every pair of
neighboring routers become adjacent.
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RFC 2178 OSPF Version 2 July 1997
Link state advertisement
Unit of data describing the local state of a router or network.
For a router, this includes the state of the router's interfaces
and adjacencies. Each link state advertisement is flooded
throughout the routing domain. The collected link state
advertisements of all routers and networks forms the protocol's
link state database. Throughout this memo, link state
advertisement is abbreviated as LSA.
Hello Protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On broadcast networks the Hello Protocol
can also dynamically discover neighboring routers.
Flooding
The part of the OSPF protocol that distributes and synchronizes
the link-state database between OSPF routers.
Designated Router
Each broadcast and NBMA network that has at least two attached
routers has a Designated Router. The Designated Router generates
an LSA for the network and has other special responsibilities in
the running of the protocol. The Designated Router is elected by
the Hello Protocol.
The Designated Router concept enables a reduction in the number of
adjacencies required on a broadcast or NBMA network. This in turn
reduces the amount of routing protocol traffic and the size of the
link-state database.
Lower-level protocols
The underlying network access protocols that provide services to
the Internet Protocol and in turn the OSPF protocol. Examples of
these are the X.25 packet and frame levels for X.25 PDNs, and the
ethernet data link layer for ethernets.
1.3. Brief history of link-state routing technology
OSPF is a link state routing protocol. Such protocols are also
referred to in the literature as SPF-based or distributed-database
protocols. This section gives a brief description of the
developments in link-state technology that have influenced the OSPF
protocol.
The first link-state routing protocol was developed for use in the
ARPANET packet switching network. This protocol is described in
[Ref3]. It has formed the starting point for all other link-state
protocols. The homogeneous ARPANET environment, i.e., single-vendor
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RFC 2178 OSPF Version 2 July 1997
packet switches connected by synchronous serial lines, simplified the
design and implementation of the original protocol.
Modifications to this protocol were proposed in [Ref4]. These
modifications dealt with increasing the fault tolerance of the
routing protocol through, among other things, adding a checksum to
the LSAs (thereby detecting database corruption). The paper also
included means for reducing the routing traffic overhead in a link-
state protocol. This was accomplished by introducing mechanisms
which enabled the interval between LSA originations to be increased
by an order of magnitude.
A link-state algorithm has also been proposed for use as an ISO IS-IS
routing protocol. This protocol is described in [Ref2]. The
protocol includes methods for data and routing traffic reduction when
operating over broadcast networks. This is accomplished by election
of a Designated Router for each broadcast network, which then
originates an LSA for the network.
The OSPF Working Group of the IETF has extended this work in
developing the OSPF protocol. The Designated Router concept has been
greatly enhanced to further reduce the amount of routing traffic
required. Multicast capabilities are utilized for additional routing
bandwidth reduction. An area routing scheme has been developed
enabling information hiding/protection/reduction. Finally, the
algorithms have been tailored for efficient operation in TCP/IP
internets.
1.4. Organization of this document
The first three sections of this specification give a general
overview of the protocol's capabilities and functions. Sections 4-16
explain the protocol's mechanisms in detail. Packet formats,
protocol constants and configuration items are specified in the
appendices.
Labels such as HelloInterval encountered in the text refer to
protocol constants. They may or may not be configurable.
Architectural constants are summarized in Appendix B. Configurable
constants are summarized in Appendix C.
The detailed specification of the protocol is presented in terms of
data structures. This is done in order to make the explanation more
precise. Implementations of the protocol are required to support the
functionality described, but need not use the precise data structures
that appear in this memo.
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RFC 2178 OSPF Version 2 July 19971.5. Acknowledgments
The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui Zhang
and the rest of the OSPF Working Group for the ideas and support they
have given to this project.
The OSPF Point-to-MultiPoint interface is based on work done by Fred
Baker.
The OSPF Cryptographic Authentication option was developed by Fred
Baker and Ran Atkinson.
2. The Link-state Database: organization and calculations
The following subsections describe the organization of OSPF's link-
state database, and the routing calculations that are performed on
the database in order to produce a router's routing table.
2.1. Representation of routers and networks
The Autonomous System's link-state database describes a directed
graph. The vertices of the graph consist of routers and networks. A
graph edge connects two routers when they are attached via a physical
point-to-point network. An edge connecting a router to a network
indicates that the router has an interface on the network. Networks
can be either transit or stub networks. Transit networks are those
capable of carrying data traffic that is neither locally originated
nor locally destined. A transit network is represented by a graph
vertex having both incoming and outgoing edges. A stub network's
vertex has only incoming edges.
The neighborhood of each network node in the graph depends on the
network's type (point-to-point, broadcast, NBMA or Point-to-
MultiPoint) and the number of routers having an interface to the
network. Three cases are depicted in Figure 1a. Rectangles indicate
routers. Circles and oblongs indicate networks. Router names are
prefixed with the letters RT and network names with the letter N.
Router interface names are prefixed by the letter I. Lines between
routers indicate point-to-point networks. The left side of the
figure shows networks with their connected routers, with the
resulting graphs shown on the right.
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RFC 2178 OSPF Version 2 July 1997
subnet can be assigned to the point-to-point network. In this case,
both routers advertise a stub link to the IP subnet, instead of
advertising each others' IP interface addresses.
The middle of Figure 1a shows a network with only one attached router
(i.e., a stub network). In this case, the network appears on the end
of a stub connection in the link-state database's graph.
When multiple routers are attached to a broadcast network, the link-
state database graph shows all routers bidirectionally connected to
the network vertex. This is pictured at the bottom of Figure 1a.
Each network (stub or transit) in the graph has an IP address and
associated network mask. The mask indicates the number of nodes on
the network. Hosts attached directly to routers (referred to as host
routes) appear on the graph as stub networks. The network mask for a
host route is always 0xffffffff, which indicates the presence of a
single node.
2.1.1. Representation of non-broadcast networks
As mentioned previously, OSPF can run over non-broadcast networks in
one of two modes: NBMA or Point-to-MultiPoint. The choice of mode
determines the way that the Hello protocol and flooding work over the
non-broadcast network, and the way that the network is represented in
the link-state database.
In NBMA mode, OSPF emulates operation over a broadcast network: a
Designated Router is elected for the NBMA network, and the Designated
Router originates an LSA for the network. The graph representation
for broadcast networks and NBMA networks is identical. This
representation is pictured in the middle of Figure 1a.
NBMA mode is the most efficient way to run OSPF over non-broadcast
networks, both in terms of link-state database size and in terms of
the amount of routing protocol traffic. However, it has one
significant restriction: it requires all routers attached to the NBMA
network to be able to communicate directly. This restriction may be
met on some non-broadcast networks, such as an ATM subnet utilizing
SVCs. But it is often not met on other non-broadcast networks, such
as PVC-only Frame Relay networks. On non-broadcast networks where not
all routers can communicate directly you can break the non-broadcast
network into logical subnets, with the routers on each subnet being
able to communicate directly, and then run each separate subnet as an
NBMA network (see [Ref15]). This however requires quite a bit of
administrative overhead, and is prone to misconfiguration. It is
probably better to run such a non-broadcast network in Point-to-
Multipoint mode.
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RFC 2178 OSPF Version 2 July 1997
In Point-to-MultiPoint mode, OSPF treats all router-to-router
connections over the non-broadcast network as if they were point-to-
point links. No Designated Router is elected for the network, nor is
there an LSA generated for the network. In fact, a vertex for the
Point-to-MultiPoint network does not appear in the graph of the
link-state database.
Figure 1b illustrates the link-state database representation of a
Point-to-MultiPoint network. On the left side of the figure, a
Point-to-MultiPoint network is pictured. It is assumed that all
routers can communicate directly, except for routers RT4 and RT5. I3
though I6 indicate the routers' IP interface addresses on the Point-
to-MultiPoint network. In the graphical representation of the link-
state database, routers that can communicate directly over the
Point-to-MultiPoint network are joined by bidirectional edges, and
each router also has a stub connection to its own IP interface
address (which is in contrast to the representation of real point-
to-point links; see Figure 1a).
On some non-broadcast networks, use of Point-to-MultiPoint mode and
data-link protocols such as Inverse ARP (see [Ref14]) will allow
autodiscovery of OSPF neighbors even though broadcast support is not
available.
2.1.2. An example link-state database
Figure 2 shows a sample map of an Autonomous System. The rectangle
labelled H1 indicates a host, which has a SLIP connection to Router
RT12. Router RT12 is therefore advertising a host route. Lines
between routers indicate physical point-to-point networks. The only
point-to-point network that has been assigned interface addresses is
the one joining Routers RT6 and RT10. Routers RT5 and RT7 have BGP
connections to other Autonomous Systems. A set of BGP-learned routes
have been displayed for both of these routers.
A cost is associated with the output side of each router interface.
This cost is configurable by the system administrator. The lower the
cost,the more likely the interface is to be used to forward data
traffic. Costs are also associated with the externally derived
routing data (e.g., the BGP-learned routes).
The directed graph resulting from the map in Figure 2 is depicted in
Figure 3. Arcs are labelled with the cost of the corresponding
router output interface. Arcs having no labelled cost have a cost of
0. Note that arcs leading from networks to routers always have cost
0; they are significant nonetheless. Note also that the externally
derived routing data appears on the graph as stubs.
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RFC 2178 OSPF Version 2 July 1997
Note that the LSA for Network N6 is actually generated by one of the
network's attached routers: the router that has been elected
Designated Router for the network.
2.2. The shortest-path tree
When no OSPF areas are configured, each router in the Autonomous
System has an identical link-state database, leading to an identical
graphical representation. A router generates its routing table from
this graph by calculating a tree of shortest paths with the router
itself as root. Obviously, the shortest- path tree depends on the
router doing the calculation. The shortest-path tree for Router RT6
in our example is depicted in Figure 5.
The tree gives the entire path to any destination network or host.
However, only the next hop to the destination is used in the
forwarding process. Note also that the best route to any router has
also been calculated. For the processing of external data, we note
the next hop and distance to any router advertising external routes.
The resulting routing table for Router RT6 is pictured in Table 2.
Note that there is a separate route for each end of a numbered
point-to-point network (in this case, the serial line between Routers
RT6 and RT10).
**FROM** **FROM**
|RT12|N9|N10|H1| |RT9|RT11|RT12|N9|
* -------------------- * ----------------------
* RT12| | | | | * RT9| | | |0 |
T N9|1 | | | | T RT11| | | |0 |
O N10|2 | | | | O RT12| | | |0 |
* H1|10 | | | | * N9| | | | |
* *
RT12's router-LSA N9's network-LSA
Figure 4: Individual link state components
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
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RFC 2178 OSPF Version 2 July 1997
Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8
Table 2: The portion of Router RT6's routing table listing local
destinations.
Routes to networks belonging to other AS'es (such as N12) appear as
dashed lines on the shortest path tree in Figure 5. Use of this
externally derived routing information is considered in the next
section.
2.3. Use of external routing information
After the tree is created the external routing information is
examined. This external routing information may originate from
another routing protocol such as BGP, or be statically configured
(static routes). Default routes can also be included as part of the
Autonomous System's external routing information.
External routing information is flooded unaltered throughout the AS.
In our example, all the routers in the Autonomous System know that
Router RT7 has two external routes, with metrics 2 and 9.
OSPF supports two types of external metrics. Type 1 external metrics
are expressed in the same units as OSPF interface cost (i.e., in
terms of the link state metric). Type 2 external metrics are an
order of magnitude larger; any Type 2 metric is considered greater
than the cost of any path internal to the AS. Use of Type 2 external
metrics assumes that routing between AS'es is the major cost of
routing a packet, and eliminates the need for conversion of external
costs to internal link state metrics.
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As an example of Type 1 external metric processing, suppose that the
Routers RT7 and RT5 in Figure 2 are advertising Type 1 external
metrics. For each advertised external route, the total cost from
Router RT6 is calculated as the sum of the external route's
advertised cost and the distance from Router RT6 to the advertising
router. When two routers are advertising the same external
destination, RT6 picks the advertising router providing the minimum
total cost. RT6 then sets the next hop to the external destination
equal to the next hop that would be used when routing packets to the
chosen advertising router.
In Figure 2, both Router RT5 and RT7 are advertising an external
route to destination Network N12. Router RT7 is preferred since it
is advertising N12 at a distance of 10 (8+2) to Router RT6, which is
better than Router RT5's 14 (6+8). Table 3 shows the entries that
are added to the routing table when external routes are examined:
Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17
Table 3: The portion of Router RT6's routing table
listing external destinations.
Processing of Type 2 external metrics is simpler. The AS boundary
router advertising the smallest external metric is chosen, regardless
of the internal distance to the AS boundary router. Suppose in our
example both Router RT5 and Router RT7 were advertising Type 2
external routes. Then all traffic destined for Network N12 would be
forwarded to Router RT7, since 2 < 8. When several equal-cost Type 2
routes exist, the internal distance to the advertising routers is
used to break the tie.
Both Type 1 and Type 2 external metrics can be present in the AS at
the same time. In that event, Type 1 external metrics always take
precedence.
This section has assumed that packets destined for external
destinations are always routed through the advertising AS boundary
router. This is not always desirable. For example, suppose in
Figure 2 there is an additional router attached to Network N6, called
Router RTX. Suppose further that RTX does not participate in OSPF
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RFC 2178 OSPF Version 2 July 1997
routing, but does exchange BGP information with the AS boundary
router RT7. Then, Router RT7 would end up advertising OSPF external
routes for all destinations that should be routed to RTX. An extra
hop will sometimes be introduced if packets for these destinations
need always be routed first to Router RT7 (the advertising router).
To deal with this situation, the OSPF protocol allows an AS boundary
router to specify a "forwarding address" in its AS- external-LSAs. In
the above example, Router RT7 would specify RTX's IP address as the
"forwarding address" for all those destinations whose packets should
be routed directly to RTX.
The "forwarding address" has one other application. It enables
routers in the Autonomous System's interior to function as "route
servers". For example, in Figure 2 the router RT6 could become a
route server, gaining external routing information through a
combination of static configuration and external routing protocols.
RT6 would then start advertising itself as an AS boundary router, and
would originate a collection of OSPF AS-external-LSAs. In each AS-
external-LSA, Router RT6 would specify the correct Autonomous System
exit point to use for the destination through appropriate setting of
the LSA's "forwarding address" field.
2.4. Equal-cost multipath
The above discussion has been simplified by considering only a single
route to any destination. In reality, if multiple equal-cost routes
to a destination exist, they are all discovered and used. This
requires no conceptual changes to the algorithm, and its discussion
is postponed until we consider the tree-building process in more
detail.
With equal cost multipath, a router potentially has several available
next hops towards any given destination.
3. Splitting the AS into Areas
OSPF allows collections of contiguous networks and hosts to be
grouped together. Such a group, together with the routers having
interfaces to any one of the included networks, is called an area.
Each area runs a separate copy of the basic link-state routing
algorithm. This means that each area has its own link-state database
and corresponding graph, as explained in the previous section.
The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic
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RFC 2178 OSPF Version 2 July 1997
as compared to treating the entire Autonomous System as a single
link-state domain.
With the introduction of areas, it is no longer true that all routers
in the AS have an identical link-state database. A router actually
has a separate link-state database for each area it is connected to.
(Routers connected to multiple areas are called area border routers).
Two routers belonging to the same area have, for that area, identical
area link-state databases.
Routing in the Autonomous System takes place on two levels, depending
on whether the source and destination of a packet reside in the same
area (intra-area routing is used) or different areas (inter-area
routing is used). In intra-area routing, the packet is routed solely
on information obtained within the area; no routing information
obtained from outside the area can be used. This protects intra-area
routing from the injection of bad routing information. We discuss
inter-area routing in Section 3.2.
3.1. The backbone of the Autonomous System
The OSPF backbone is the special OSPF Area 0 (often written as Area
0.0.0.0, since OSPF Area ID's are typically formatted as IP
addresses). The OSPF backbone always contains all area border
routers. The backbone is responsible for distributing routing
information between non-backbone areas. The backbone must be
contiguous. However, it need not be physically contiguous; backbone
connectivity can be established/maintained through the configuration
of virtual links.
Virtual links can be configured between any two backbone routers that
have an interface to a common non-backbone area. Virtual links
belong to the backbone. The protocol treats two routers joined by a
virtual link as if they were connected by an unnumbered point-to-
point backbone network. On the graph of the backbone, two such
routers are joined by arcs whose costs are the intra-area distances
between the two routers. The routing protocol traffic that flows
along the virtual link uses intra-area routing only.
3.2. Inter-area routing
When routing a packet between two non-backbone areas the backbone is
used. The path that the packet will travel can be broken up into
three contiguous pieces: an intra-area path from the source to an
area border router, a backbone path between the source and
destination areas, and then another intra-area path to the
destination. The algorithm finds the set of such paths that have the
smallest cost.
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Looking at this another way, inter-area routing can be pictured as
forcing a star configuration on the Autonomous System, with the
backbone as hub and each of the non-backbone areas as spokes.
The topology of the backbone dictates the backbone paths used between
areas. The topology of the backbone can be enhanced by adding
virtual links. This gives the system administrator some control over
the routes taken by inter-area traffic.
The correct area border router to use as the packet exits the source
area is chosen in exactly the same way routers advertising external
routes are chosen. Each area border router in an area summarizes for
the area its cost to all networks external to the area. After the
SPF tree is calculated for the area, routes to all inter-area
destinations are calculated by examining the summaries of the area
border routers.
3.3. Classification of routers
Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as Router RT5 in Figure 2. When the AS is split
into OSPF areas, the routers are further divided according to
function into the following four overlapping categories:
Internal routers
A router with all directly connected networks belonging to the
same area. These routers run a single copy of the basic routing
algorithm.
Area border routers
A router that attaches to multiple areas. Area border routers run
multiple copies of the basic algorithm, one copy for each attached
area. Area border routers condense the topological information of
their attached areas for distribution to the backbone. The
backbone in turn distributes the information to the other areas.
Backbone routers
A router that has an interface to the backbone area. This
includes all routers that interface to more than one area (i.e.,
area border routers). However, backbone routers do not have to be
area border routers. Routers with all interfaces connecting to
the backbone area are supported.
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AS boundary routers
A router that exchanges routing information with routers belonging
to other Autonomous Systems. Such a router advertises AS external
routing information throughout the Autonomous System. The paths
to each AS boundary router are known by every router in the AS.
This classification is completely independent of the previous
classifications: AS boundary routers may be internal or area
border routers, and may or may not participate in the backbone.
3.4. A sample area configuration
Figure 6 shows a sample area configuration. The first area consists
of networks N1-N4, along with their attached routers RT1-RT4. The
second area consists of networks N6-N8, along with their attached
routers RT7, RT8, RT10 and RT11. The third area consists of networks
N9-N11 and Host H1, along with their attached routers RT9, RT11 and
RT12. The third area has been configured so that networks N9-N11 and
Host H1 will all be grouped into a single route, when advertised
external to the area (see Section 3.5 for more details).
In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area
border routers. Finally, as before, Routers RT5 and RT7 are AS
boundary routers.
Figure 7 shows the resulting link-state database for the Area 1. The
figure completely describes that area's intra-area routing.
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It also shows the complete view of the internet for the two internal
routers RT1 and RT2. It is the job of the area border routers, RT3
and RT4, to advertise into Area 1 the distances to all destinations
external to the area. These are indicated in Figure 7 by the dashed
stub routes. Also, RT3 and RT4 must advertise into Area 1 the
location of the AS boundary routers RT5 and RT7. Finally, AS-
external-LSAs from RT5 and RT7 are flooded throughout the entire AS,
and in particular throughout Area 1. These LSAs are included in Area
1's database, and yield routes to Networks N12-N15.
Routers RT3 and RT4 must also summarize Area 1's topology for
distribution to the backbone. Their backbone LSAs are shown in Table
4. These summaries show which networks are contained in Area 1
(i.e., Networks N1-N4), and the distance to these networks from the
routers RT3 and RT4 respectively.
The link-state database for the backbone is shown in Figure 8. The
set of routers pictured are the backbone routers. Router RT11 is a
backbone router because it belongs to two areas. In order to make
the backbone connected, a virtual link has been configured between
Routers R10 and R11.
The area border routers RT3, RT4, RT7, RT10 and RT11 condense the
routing information of their attached non-backbone areas for
distribution via the backbone; these are the dashed stubs that appear
in Figure 8. Remember that the third area has been configured to
condense Networks N9-N11 and Host H1 into a single route. This
yields a single dashed line for networks N9-N11 and Host H1 in Figure
8. Routers RT5 and RT7 are AS boundary routers; their externally
derived information also appears on the graph in Figure 8 as stubs.
Network RT3 adv. RT4 adv.
_____________________________
N1 4 4
N2 4 4
N3 1 1
N4 2 3
Table 4: Networks advertised to the backbone
by Routers RT3 and RT4.
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Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised internally
to the area by RT3 and RT4. The advertisements that Router RT3 and
RT4 will make into Area 1 are shown in Table 6. Note that Table 6
assumes that an area range has been configured for the backbone which
groups Ia and Ib into a single LSA.
The information imported into Area 1 by Routers RT3 and RT4 enables
an internal router, such as RT1, to choose an area border router
intelligently. Router RT1 would use RT4 for traffic to Network N6,
RT3 for traffic to Network N10, and would load share between the two
for traffic to Network N8.
dist from dist from
RT3 RT4
__________________________________
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
__________________________________
to Ia 20 27
to Ib 15 22
__________________________________
to RT5 14 8
to RT7 20 14
Table 5: Backbone distances calculated
by Routers RT3 and RT4.
Destination RT3 adv. RT4 adv.
_________________________________
Ia,Ib 20 27
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 29 36
_________________________________
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.
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Router RT1 can also determine in this manner the shortest path to the
AS boundary routers RT5 and RT7. Then, by looking at RT5 and RT7's
AS-external-LSAs, Router RT1 can decide between RT5 or RT7 when
sending to a destination in another Autonomous System (one of the
networks N12-N15).
Note that a failure of the line between Routers RT6 and RT10 will
cause the backbone to become disconnected. Configuring a virtual
link between Routers RT7 and RT10 will give the backbone more
connectivity and more resistance to such failures.
3.5. IP subnetting support
OSPF attaches an IP address mask to each advertised route. The mask
indicates the range of addresses being described by the particular
route. For example, a summary-LSA for the destination 128.185.0.0
with a mask of 0xffff0000 actually is describing a single route to
the collection of destinations 128.185.0.0 - 128.185.255.255.
Similarly, host routes are always advertised with a mask of
0xffffffff, indicating the presence of only a single destination.
Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-length
subnetting. This means that a single IP class A, B, or C network
number can be broken up into many subnets of various sizes. For
example, the network 128.185.0.0 could be broken up into 62
variable-sized subnets: 15 subnets of size 4K, 15 subnets of size
256, and 32 subnets of size 8. Table 7 shows some of the resulting
network addresses together with their masks.
Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 0xfffffff8 8
Table 7: Some sample subnet sizes.
There are many possible ways of dividing up a class A, B, and C
network into variable sized subnets. The precise procedure for doing
so is beyond the scope of this specification. This specification
however establishes the following guideline: When an IP packet is
forwarded, it is always forwarded to the network that is the best
match for the packet's destination. Here best match is synonymous
with the longest or most specific match. For example, the default
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route with destination of 0.0.0.0 and mask 0x00000000 is always a
match for every IP destination. Yet it is always less specific than
any other match. Subnet masks must be assigned so that the best
match for any IP destination is unambiguous.
Attaching an address mask to each route also enables the support of
IP supernetting. For example, a single physical network segment could
be assigned the [address,mask] pair [192.9.4.0,0xfffffc00]. The
segment would then be single IP network, containing addresses from
the four consecutive class C network numbers 192.9.4.0 through
192.9.7.0. Such addressing is now becoming commonplace with the
advent of CIDR (see [Ref10]).
In order to get better aggregation at area boundaries, area address
ranges can be employed (see Section C.2 for more details). Each
address range is defined as an [address,mask] pair. Many separate
networks may then be contained in a single address range, just as a
subnetted network is composed of many separate subnets. Area border
routers then summarize the area contents (for distribution to the
backbone) by advertising a single route for each address range. The
cost of the route is the maximum cost to any of the networks falling
in the specified range.
For example, an IP subnetted network might be configured as a single
OSPF area. In that case, a single address range could be configured:
a class A, B, or C network number along with its natural IP mask.
Inside the area, any number of variable sized subnets could be
defined. However, external to the area a single route for the entire
subnetted network would be distributed, hiding even the fact that the
network is subnetted at all. The cost of this route is the maximum
of the set of costs to the component subnets.
3.6. Supporting stub areas
In some Autonomous Systems, the majority of the link-state database
may consist of AS-external-LSAs. An OSPF AS-external-LSA is usually
flooded throughout the entire AS. However, OSPF allows certain areas
to be configured as "stub areas". AS-external-LSAs are not flooded
into/throughout stub areas; routing to AS external destinations in
these areas is based on a (per-area) default only. This reduces the
link-state database size, and therefore the memory requirements, for
a stub area's internal routers.
In order to take advantage of the OSPF stub area support, default
routing must be used in the stub area. This is accomplished as
follows. One or more of the stub area's area border routers must
advertise a default route into the stub area via summary-LSAs. These
summary defaults are flooded throughout the stub area, but no
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further. (For this reason these defaults pertain only to the
particular stub area). These summary default routes will be used for
any destination that is not explicitly reachable by an intra-area or
inter-area path (i.e., AS external destinations).
An area can be configured as a stub when there is a single exit point
from the area, or when the choice of exit point need not be made on a
per-external-destination basis. For example, Area 3 in Figure 6
could be configured as a stub area, because all external traffic must
travel though its single area border router RT11. If Area 3 were
configured as a stub, Router RT11 would advertise a default route for
distribution inside Area 3 (in a summary-LSA), instead of flooding
the AS-external-LSAs for Networks N12-N15 into/throughout the area.
The OSPF protocol ensures that all routers belonging to an area agree
on whether the area has been configured as a stub. This guarantees
that no confusion will arise in the flooding of AS-external-LSAs.
There are a couple of restrictions on the use of stub areas. Virtual
links cannot be configured through stub areas. In addition, AS
boundary routers cannot be placed internal to stub areas.
3.7. Partitions of areas
OSPF does not actively attempt to repair area partitions. When an
area becomes partitioned, each component simply becomes a separate
area. The backbone then performs routing between the new areas.
Some destinations reachable via intra-area routing before the
partition will now require inter-area routing.
However, in order to maintain full routing after the partition, an
address range must not be split across multiple components of the
area partition. Also, the backbone itself must not partition. If it
does, parts of the Autonomous System will become unreachable.
Backbone partitions can be repaired by configuring virtual links (see
Section 15).
Another way to think about area partitions is to look at the
Autonomous System graph that was introduced in Section 2. Area IDs
can be viewed as colors for the graph's edges.[1] Each edge of the
graph connects to a network, or is itself a point-to-point network.
In either case, the edge is colored with the network's Area ID.
A group of edges, all having the same color, and interconnected by
vertices, represents an area. If the topology of the Autonomous
System is intact, the graph will have several regions of color, each
color being a distinct Area ID.
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When the AS topology changes, one of the areas may become
partitioned. The graph of the AS will then have multiple regions of
the same color (Area ID). The routing in the Autonomous System will
continue to function as long as these regions of same color are
connected by the single backbone region.
4. Functional Summary
A separate copy of OSPF's basic routing algorithm runs in each area.
Routers having interfaces to multiple areas run multiple copies of
the algorithm. A brief summary of the routing algorithm follows.
When a router starts, it first initializes the routing protocol data
structures. The router then waits for indications from the lower-
level protocols that its interfaces are functional.
A router then uses the OSPF's Hello Protocol to acquire neighbors.
The router sends Hello packets to its neighbors, and in turn receives
their Hello packets. On broadcast and point-to-point networks, the
router dynamically detects its neighboring routers by sending its
Hello packets to the multicast address AllSPFRouters. On non-
broadcast networks, some configuration information may be necessary
in order to discover neighbors. On broadcast and NBMA networks the
Hello Protocol also elects a Designated router for the network.
The router will attempt to form adjacencies with some of its newly
acquired neighbors. Link-state databases are synchronized between
pairs of adjacent routers. On broadcast and NBMA networks, the
Designated Router determines which routers should become adjacent.
Adjacencies control the distribution of routing information. Routing
updates are sent and received only on adjacencies.
A router periodically advertises its state, which is also called link
state. Link state is also advertised when a router's state changes.
A router's adjacencies are reflected in the contents of its LSAs.
This relationship between adjacencies and link state allows the
protocol to detect dead routers in a timely fashion.
LSAs are flooded throughout the area. The flooding algorithm is
reliable, ensuring that all routers in an area have exactly the same
link-state database. This database consists of the collection of
LSAs originated by each router belonging to the area. From this
database each router calculates a shortest-path tree, with itself as
root. This shortest-path tree in turn yields a routing table for the
protocol.
Moy Standards Track [Page 34]

RFC 2178 OSPF Version 2 July 19974.1. Inter-area routing
The previous section described the operation of the protocol within a
single area. For intra-area routing, no other routing information is
pertinent. In order to be able to route to destinations outside of
the area, the area border routers inject additional routing
information into the area. This additional information is a
distillation of the rest of the Autonomous System's topology.
This distillation is accomplished as follows: Each area border router
is by definition connected to the backbone. Each area border router
summarizes the topology of its attached non-backbone areas for
transmission on the backbone, and hence to all other area border
routers. An area border router then has complete topological
information concerning the backbone, and the area summaries from each
of the other area border routers. From this information, the router
calculates paths to all inter-area destinations. The router then
advertises these paths into its attached areas. This enables the
area's internal routers to pick the best exit router when forwarding
traffic inter-area destinations.
4.2. AS external routes
Routers that have information regarding other Autonomous Systems can
flood this information throughout the AS. This external routing
information is distributed verbatim to every participating router.
There is one exception: external routing information is not flooded
into "stub" areas (see Section 3.6).
To utilize external routing information, the path to all routers
advertising external information must be known throughout the AS
(excepting the stub areas). For that reason, the locations of these
AS boundary routers are summarized by the (non-stub) area border
routers.
4.3. Routing protocol packets
The OSPF protocol runs directly over IP, using IP protocol 89. OSPF
does not provide any explicit fragmentation/reassembly support. When
fragmentation is necessary, IP fragmentation/reassembly is used.
OSPF protocol packets have been designed so that large protocol
packets can generally be split into several smaller protocol packets.
This practice is recommended; IP fragmentation should be avoided
whenever possible.
Routing protocol packets should always be sent with the IP TOS field
set to 0. If at all possible, routing protocol packets should be
given preference over regular IP data traffic, both when being sent
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RFC 2178 OSPF Version 2 July 1997
and received. As an aid to accomplishing this, OSPF protocol packets
should have their IP precedence field set to the value Internetwork
Control (see [Ref5]).
All OSPF protocol packets share a common protocol header that is
described in Appendix A. The OSPF packet types are listed below in
Table 8. Their formats are also described in Appendix A.
Type Packet name
Protocol function
__________________________________________________________
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment
Table 8: OSPF packet types.
OSPF's Hello protocol uses Hello packets to discover and maintain
neighbor relationships. The Database Description and Link State
Request packets are used in the forming of adjacencies. OSPF's
reliable update mechanism is implemented by the Link State Update and
Link State Acknowledgment packets.
Each Link State Update packet carries a set of new link state
advertisements (LSAs) one hop further away from their point of
origination. A single Link State Update packet may contain the LSAs
of several routers. Each LSA is tagged with the ID of the
originating router and a checksum of its link state contents. Each
LSA also has a type field; the different types of OSPF LSAs are
listed below in Table 9.
OSPF routing packets (with the exception of Hellos) are sent only
over adjacencies. This means that all OSPF protocol packets travel a
single IP hop, except those that are sent over virtual adjacencies.
The IP source address of an OSPF protocol packet is one end of a
router adjacency, and the IP destination address is either the other
end of the adjacency or an IP multicast address.
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LS LSA LSA description
type name
________________________________________________________
1 Router-LSAs Originated by all routers.
This LSA describes
the collected states of the
router's interfaces to an
area. Flooded throughout a
single area only.
________________________________________________________
2 Network-LSAs Originated for broadcast
and NBMA networks by
the Designated Router. This
LSA contains the
list of routers connected
to the network. Flooded
throughout a single area only.
________________________________________________________
3,4 Summary-LSAs Originated by area border
routers, and flooded through-
out the LSA's associated
area. Each summary-LSA
describes a route to a
destination outside the area,
yet still inside the AS
(i.e., an inter-area route).
Type 3 summary-LSAs describe
routes to networks. Type 4
summary-LSAs describe
routes to AS boundary routers.
________________________________________________________
5 AS-external-LSAs Originated by AS boundary
routers, and flooded through-
out the AS. Each
AS-external-LSA describes
a route to a destination in
another Autonomous System.
Default routes for the AS can
also be described by
AS-external-LSAs.
Table 9: OSPF link state advertisements (LSAs).
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RFC 2178 OSPF Version 2 July 19974.4. Basic implementation requirements
An implementation of OSPF requires the following pieces of system
support:
Timers
Two different kind of timers are required. The first kind, called
"single shot timers", fire once and cause a protocol event to be
processed. The second kind, called "interval timers", fire at
continuous intervals. These are used for the sending of packets
at regular intervals. A good example of this is the regular
broadcast of Hello packets. The granularity of both kinds of
timers is one second.
Interval timers should be implemented to avoid drift. In some
router implementations, packet processing can affect timer
execution. When multiple routers are attached to a single
network, all doing broadcasts, this can lead to the
synchronization of routing packets (which should be avoided). If
timers cannot be implemented to avoid drift, small random amounts
should be added to/subtracted from the interval timer at each
firing.
IP multicast
Certain OSPF packets take the form of IP multicast datagrams.
Support for receiving and sending IP multicast datagrams, along
with the appropriate lower-level protocol support, is required.
The IP multicast datagrams used by OSPF never travel more than one
hop. For this reason, the ability to forward IP multicast
datagrams is not required. For information on IP multicast, see
[Ref7].
Variable-length subnet support
The router's IP protocol support must include the ability to
divide a single IP class A, B, or C network number into many
subnets of various sizes. This is commonly called variable-length
subnetting; see Section 3.5 for details.
IP supernetting support
The router's IP protocol support must include the ability to
aggregate contiguous collections of IP class A, B, and C networks
into larger quantities called supernets. Supernetting has been
proposed as one way to improve the scaling of IP routing in the
worldwide Internet. For more information on IP supernetting, see
[Ref10].
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Lower-level protocol support
The lower level protocols referred to here are the network access
protocols, such as the Ethernet data link layer. Indications must
be passed from these protocols to OSPF as the network interface
goes up and down. For example, on an ethernet it would be
valuable to know when the ethernet transceiver cable becomes
unplugged.
Non-broadcast lower-level protocol support
On non-broadcast networks, the OSPF Hello Protocol can be aided by
providing an indication when an attempt is made to send a packet
to a dead or non-existent router. For example, on an X.25 PDN a
dead neighboring router may be indicated by the reception of a
X.25 clear with an appropriate cause and diagnostic, and this
information would be passed to OSPF.
List manipulation primitives
Much of the OSPF functionality is described in terms of its
operation on lists of LSAs. For example, the collection of LSAs
that will be retransmitted to an adjacent router until
acknowledged are described as a list. Any particular LSA may be
on many such lists. An OSPF implementation needs to be able to
manipulate these lists, adding and deleting constituent LSAs as
necessary.
Tasking support
Certain procedures described in this specification invoke other
procedures. At times, these other procedures should be executed
in-line, that is, before the current procedure is finished. This
is indicated in the text by instructions to execute a procedure.
At other times, the other procedures are to be executed only when
the current procedure has finished. This is indicated by
instructions to schedule a task.
4.5. Optional OSPF capabilities
The OSPF protocol defines several optional capabilities. A router
indicates the optional capabilities that it supports in its OSPF
Hello packets, Database Description packets and in its LSAs. This
enables routers supporting a mix of optional capabilities to coexist
in a single Autonomous System.
Some capabilities must be supported by all routers attached to a
specific area. In this case, a router will not accept a neighbor's
Hello Packet unless there is a match in reported capabilities (i.e.,
a capability mismatch prevents a neighbor relationship from forming).
An example of this is the ExternalRoutingCapability (see below).
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Other capabilities can be negotiated during the Database Exchange
process. This is accomplished by specifying the optional
capabilities in Database Description packets. A capability mismatch
with a neighbor in this case will result in only a subset of the link
state database being exchanged between the two neighbors.
The routing table build process can also be affected by the
presence/absence of optional capabilities. For example, since the
optional capabilities are reported in LSAs, routers incapable of
certain functions can be avoided when building the shortest path
tree.
The OSPF optional capabilities defined in this memo are listed below.
See Section A.2 for more information.
ExternalRoutingCapability
Entire OSPF areas can be configured as "stubs" (see Section 3.6).
AS-external-LSAs will not be flooded into stub areas. This
capability is represented by the E-bit in the OSPF Options field
(see Section A.2). In order to ensure consistent configuration of
stub areas, all routers interfacing to such an area must have the
E-bit clear in their Hello packets (see Sections 9.5 and 10.5).
5. Protocol Data Structures
The OSPF protocol is described herein in terms of its operation on
various protocol data structures. The following list comprises the
top-level OSPF data structures. Any initialization that needs to be
done is noted. OSPF areas, interfaces and neighbors also have
associated data structures that are described later in this
specification.
Router ID
A 32-bit number that uniquely identifies this router in the AS.
One possible implementation strategy would be to use the smallest
IP interface address belonging to the router. If a router's OSPF
Router ID is changed, the router's OSPF software should be
restarted before the new Router ID takes effect. In this case the
router should flush its self-originated LSAs from the routing
domain (see Section 14.1) before restarting, or they will persist
for up to MaxAge minutes.
Area structures
Each one of the areas to which the router is connected has its own
data structure. This data structure describes the working of the
basic OSPF algorithm. Remember that each area runs a separate
copy of the basic OSPF algorithm.
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RFC 2178 OSPF Version 2 July 1997
Backbone (area) structure
The OSPF backbone area is responsible for the dissemination of
inter-area routing information.
Virtual links configured
The virtual links configured with this router as one endpoint. In
order to have configured virtual links, the router itself must be
an area border router. Virtual links are identified by the Router
ID of the other endpoint -- which is another area border router.
These two endpoint routers must be attached to a common area,
called the virtual link's Transit area. Virtual links are part of
the backbone, and behave as if they were unnumbered point-to-point
networks between the two routers. A virtual link uses the intra-
area routing of its Transit area to forward packets. Virtual
links are brought up and down through the building of the
shortest-path trees for the Transit area.
List of external routes
These are routes to destinations external to the Autonomous
System, that have been gained either through direct experience
with another routing protocol (such as BGP), or through
configuration information, or through a combination of the two
(e.g., dynamic external information to be advertised by OSPF with
configured metric). Any router having these external routes is
called an AS boundary router. These routes are advertised by the
router into the OSPF routing domain via AS-external-LSAs.
List of AS-external-LSAs
Part of the link-state database. These have originated from the
AS boundary routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary router, some of these AS-external-LSAs have
been self-originated.
The routing table
Derived from the link-state database. Each entry in the routing
table is indexed by a destination, and contains the destination's
cost and a set of paths to use in forwarding packets to the
destination. A path is described by its type and next hop. For
more information, see Section 11.
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RFC 2178 OSPF Version 2 July 1997
The OSPF backbone is the special OSPF area responsible for
disseminating inter-area routing information.
The area link-state database consists of the collection of router-
LSAs, network-LSAs and summary-LSAs that have originated from the
area's routers. This information is flooded throughout a single area
only. The list of AS-external-LSAs (see Section 5) is also considered
to be part of each area's link-state database.
Area ID
A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
reserved for the backbone.
List of area address ranges
In order to aggregate routing information at area boundaries, area
address ranges can be employed. Each address range is specified by
an [address,mask] pair and a status indication of either Advertise
or DoNotAdvertise (see Section 12.4.3).
Associated router interfaces
This router's interfaces connecting to the area. A router
interface belongs to one and only one area (or the backbone). For
the backbone area this list includes all the virtual links. A
virtual link is identified by the Router ID of its other endpoint;
its cost is the cost of the shortest intra-area path through the
Transit area that exists between the two routers.
List of router-LSAs
A router-LSA is generated by each router in the area. It
describes the state of the router's interfaces to the area.
List of network-LSAs
One network-LSA is generated for each transit broadcast and NBMA
network in the area. A network-LSA describes the set of routers
currently connected to the network.
List of summary-LSAs
Summary-LSAs originate from the area's area border routers. They
describe routes to destinations internal to the Autonomous System,
yet external to the area (i.e., inter-area destinations).
Shortest-path tree
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router-LSAs and network-LSAs by
the Dijkstra algorithm (see Section 16.1).
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TransitCapability
This parameter indicates whether the area can carry data traffic
that neither originates nor terminates in the area itself. This
parameter is calculated when the area's shortest-path tree is
built (see Section 16.1, where TransitCapability is set to TRUE if
and only if there are one or more fully adjacent virtual links
using the area as Transit area), and is used as an input to a
subsequent step of the routing table build process (see Section16.3). When an area's TransitCapability is set to TRUE, the area
is said to be a "transit area".
ExternalRoutingCapability
Whether AS-external-LSAs will be flooded into/throughout the area.
This is a configurable parameter. If AS-external-LSAs are
excluded from the area, the area is called a "stub". Within stub
areas, routing to AS external destinations will be based solely on
a default summary route. The backbone cannot be configured as a
stub area. Also, virtual links cannot be configured through stub
areas. For more information, see Section 3.6.
StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost
indicates the cost of the default summary-LSA that the router
should advertise into the area. See Section 12.4.3 for more
information.
Unless otherwise specified, the remaining sections of this document
refer to the operation of the OSPF protocol within a single area.
7. Bringing Up Adjacencies
OSPF creates adjacencies between neighboring routers for the purpose
of exchanging routing information. Not every two neighboring routers
will become adjacent. This section covers the generalities involved
in creating adjacencies. For further details consult Section 10.
7.1. The Hello Protocol
The Hello Protocol is responsible for establishing and maintaining
neighbor relationships. It also ensures that communication between
neighbors is bidirectional. Hello packets are sent periodically out
all router interfaces. Bidirectional communication is indicated when
the router sees itself listed in the neighbor's Hello Packet. On
broadcast and NBMA networks, the Hello Protocol elects a Designated
Router for the network.
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The Hello Protocol works differently on broadcast networks, NBMA
networks and Point-to-MultiPoint networks. On broadcast networks,
each router advertises itself by periodically multicasting Hello
Packets. This allows neighbors to be discovered dynamically. These
Hello Packets contain the router's view of the Designated Router's
identity, and the list of routers whose Hello Packets have been seen
recently.
On NBMA networks some configuration information may be necessary for
the operation of the Hello Protocol. Each router that may
potentially become Designated Router has a list of all other routers
attached to the network. A router, having Designated Router
potential, sends Hello Packets to all other potential Designated
Routers when its interface to the NBMA network first becomes
operational. This is an attempt to find the Designated Router for
the network. If the router itself is elected Designated Router, it
begins sending Hello Packets to all other routers attached to the
network.
On Point-to-MultiPoint networks, a router sends Hello Packets to all
neighbors with which it can communicate directly. These neighbors may
be discovered dynamically through a protocol such as Inverse ARP (see
[Ref14]), or they may be configured.
After a neighbor has been discovered, bidirectional communication
ensured, and (if on a broadcast or NBMA network) a Designated Router
elected, a decision is made regarding whether or not an adjacency
should be formed with the neighbor (see Section 10.4). If an
adjacency is to be formed, the first step is to synchronize the
neighbors' link-state databases. This is covered in the next
section.
7.2. The Synchronization of Databases
In a link-state routing algorithm, it is very important for all
routers' link-state databases to stay synchronized. OSPF simplifies
this by requiring only adjacent routers to remain synchronized. The
synchronization process begins as soon as the routers attempt to
bring up the adjacency. Each router describes its database by
sending a sequence of Database Description packets to its neighbor.
Each Database Description Packet describes a set of LSAs belonging to
the router's database. When the neighbor sees an LSA that is more
recent than its own database copy, it makes a note that this newer
LSA should be requested.
This sending and receiving of Database Description packets is called
the "Database Exchange Process". During this process, the two
routers form a master/slave relationship. Each Database Description
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Packet has a sequence number. Database Description Packets sent by
the master (polls) are acknowledged by the slave through echoing of
the sequence number. Both polls and their responses contain
summaries of link state data. The master is the only one allowed to
retransmit Database Description Packets. It does so only at fixed
intervals, the length of which is the configured per-interface
constant RxmtInterval.
Each Database Description contains an indication that there are more
packets to follow --- the M-bit. The Database Exchange Process is
over when a router has received and sent Database Description Packets
with the M-bit off.
During and after the Database Exchange Process, each router has a
list of those LSAs for which the neighbor has more up-to-date
instances. These LSAs are requested in Link State Request Packets.
Link State Request packets that are not satisfied are retransmitted
at fixed intervals of time RxmtInterval. When the Database
Description Process has completed and all Link State Requests have
been satisfied, the databases are deemed synchronized and the routers
are marked fully adjacent. At this time the adjacency is fully
functional and is advertised in the two routers' router-LSAs.
The adjacency is used by the flooding procedure as soon as the
Database Exchange Process begins. This simplifies database
synchronization, and guarantees that it finishes in a predictable
period of time.
7.3. The Designated Router
Every broadcast and NBMA network has a Designated Router. The
Designated Router performs two main functions for the routing
protocol:
o The Designated Router originates a network-LSA on behalf of
the network. This LSA lists the set of routers (including
the Designated Router itself) currently attached to the
network. The Link State ID for this LSA (see Section12.1.4) is the IP interface address of the Designated
Router. The IP network number can then be obtained by using
the network's subnet/network mask.
o The Designated Router becomes adjacent to all other routers
on the network. Since the link state databases are
synchronized across adjacencies (through adjacency bring-up
and then the flooding procedure), the Designated Router
plays a central part in the synchronization process.
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The Designated Router is elected by the Hello Protocol. A router's
Hello Packet contains its Router Priority, which is configurable on a
per-interface basis. In general, when a router's interface to a
network first becomes functional, it checks to see whether there is
currently a Designated Router for the network. If there is, it
accepts that Designated Router, regardless of its Router Priority.
(This makes it harder to predict the identity of the Designated
Router, but ensures that the Designated Router changes less often.
See below.) Otherwise, the router itself becomes Designated Router
if it has the highest Router Priority on the network. A more
detailed (and more accurate) description of Designated Router
election is presented in Section 9.4.
The Designated Router is the endpoint of many adjacencies. In order
to optimize the flooding procedure on broadcast networks, the
Designated Router multicasts its Link State Update Packets to the
address AllSPFRouters, rather than sending separate packets over each
adjacency.
Section 2 of this document discusses the directed graph
representation of an area. Router nodes are labelled with their
Router ID. Transit network nodes are actually labelled with the IP
address of their Designated Router. It follows that when the
Designated Router changes, it appears as if the network node on the
graph is replaced by an entirely new node. This will cause the
network and all its attached routers to originate new LSAs. Until
the link-state databases again converge, some temporary loss of
connectivity may result. This may result in ICMP unreachable
messages being sent in response to data traffic. For that reason,
the Designated Router should change only infrequently. Router
Priorities should be configured so that the most dependable router on
a network eventually becomes Designated Router.
7.4. The Backup Designated Router
In order to make the transition to a new Designated Router smoother,
there is a Backup Designated Router for each broadcast and NBMA
network. The Backup Designated Router is also adjacent to all
routers on the network, and becomes Designated Router when the
previous Designated Router fails. If there were no Backup Designated
Router, when a new Designated Router became necessary, new
adjacencies would have to be formed between the new Designated Router
and all other routers attached to the network. Part of the adjacency
forming process is the synchronizing of link-state databases, which
can potentially take quite a long time. During this time, the
network would not be available for transit data traffic. The Backup
Designated obviates the need to form these adjacencies, since they
already exist. This means the period of disruption in transit
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traffic lasts only as long as it takes to flood the new LSAs (which
announce the new Designated Router).
The Backup Designated Router does not generate a network-LSA for the
network. (If it did, the transition to a new Designated Router would
be even faster. However, this is a tradeoff between database size
and speed of convergence when the Designated Router disappears.)
The Backup Designated Router is also elected by the Hello Protocol.
Each Hello Packet has a field that specifies the Backup Designated
Router for the network.
In some steps of the flooding procedure, the Backup Designated Router
plays a passive role, letting the Designated Router do more of the
work. This cuts down on the amount of local routing traffic. See
Section 13.3 for more information.
7.5. The graph of adjacencies
An adjacency is bound to the network that the two routers have in
common. If two routers have multiple networks in common, they may
have multiple adjacencies between them.
One can picture the collection of adjacencies on a network as forming
an undirected graph. The vertices consist of routers, with an edge
joining two routers if they are adjacent. The graph of adjacencies
describes the flow of routing protocol packets, and in particular
Link State Update Packets, through the Autonomous System.
Two graphs are possible, depending on whether a Designated Router is
elected for the network. On physical point-to-point networks,
Point-to-MultiPoint networks and virtual links, neighboring routers
become adjacent whenever they can communicate directly. In contrast,
on broadcast and NBMA networks only the Designated Router and the
Backup Designated Router become adjacent to all other routers
attached to the network.
These graphs are shown in Figure 10. It is assumed that Router RT7
has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated Router
performs a lesser function during the flooding procedure than the
Designated Router (see Section 13.3). This is the reason for the
dashed lines connecting the Backup Designated Router RT3.
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Version #
Set to 2, the version number of the protocol as documented in this
specification.
Packet type
The type of OSPF packet, such as Link state Update or Hello
Packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the packet).
Area ID
The OSPF area that the packet is being sent into.
Checksum
The standard IP 16-bit one's complement checksum of the entire
OSPF packet, excluding the 64-bit authentication field. This
checksum is calculated as part of the appropriate authentication
procedure; for some OSPF authentication types, the checksum
calculation is omitted. See Section D.4 for details.
AuType and Authentication
Each OSPF packet exchange is authenticated. Authentication types
are assigned by the protocol and are documented in Appendix D. A
different authentication procedure can be used for each IP
network/subnet. Autype indicates the type of authentication
procedure in use. The 64-bit authentication field is then for use
by the chosen authentication procedure. This procedure should be
the last called when forming the packet to be sent. See Section
D.4 for details.
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The IP destination address for the packet is selected as follows. On
physical point-to-point networks, the IP destination is always set to
the address AllSPFRouters. On all other network types (including
virtual links), the majority of OSPF packets are sent as unicasts,
i.e., sent directly to the other end of the adjacency. In this case,
the IP destination is just the Neighbor IP address associated with
the other end of the adjacency (see Section 10). The only packets
not sent as unicasts are on broadcast networks; on these networks
Hello packets are sent to the multicast destination AllSPFRouters,
the Designated Router and its Backup send both Link State Update
Packets and Link State Acknowledgment Packets to the multicast
address AllSPFRouters, while all other routers send both their Link
State Update and Link State Acknowledgment Packets to the multicast
address AllDRouters.
Retransmissions of Link State Update packets are ALWAYS sent as
unicasts.
The IP source address should be set to the IP address of the sending
interface. Interfaces to unnumbered point-to-point networks have no
associated IP address. On these interfaces, the IP source should be
set to any of the other IP addresses belonging to the router. For
this reason, there must be at least one IP address assigned to the
router.[2] Note that, for most purposes, virtual links act precisely
the same as unnumbered point-to-point networks. However, each
virtual link does have an IP interface address (discovered during the
routing table build process) which is used as the IP source when
sending packets over the virtual link.
For more information on the format of specific OSPF packet types,
consult the sections listed in Table 10.
Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing OSPF protocol packet transmission.
8.2. Receiving protocol packets
Whenever a protocol packet is received by the router it is marked
with the interface it was received on. For routers that have virtual
links configured, it may not be immediately obvious which interface
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to associate the packet with. For example, consider the Router RT11
depicted in Figure 6. If RT11 receives an OSPF protocol packet on
its interface to Network N8, it may want to associate the packet with
the interface to Area 2, or with the virtual link to Router RT10
(which is part of the backbone). In the following, we assume that
the packet is initially associated with the non-virtual link.[3]
In order for the packet to be accepted at the IP level, it must pass
a number of tests, even before the packet is passed to OSPF for
processing:
o The IP checksum must be correct.
o The packet's IP destination address must be the IP address
of the receiving interface, or one of the IP multicast
addresses AllSPFRouters or AllDRouters.
o The IP protocol specified must be OSPF (89).
o Locally originated packets should not be passed on to OSPF.
That is, the source IP address should be examined to make
sure this is not a multicast packet that the router itself
generated.
Next, the OSPF packet header is verified. The fields specified
in the header must match those configured for the receiving
interface. If they do not, the packet should be discarded:
o The version number field must specify protocol version 2.
o The Area ID found in the OSPF header must be verified. If
both of the following cases fail, the packet should be
discarded. The Area ID specified in the header must either:
(1) Match the Area ID of the receiving interface. In this
case, the packet has been sent over a single hop.
Therefore, the packet's IP source address is required to
be on the same network as the receiving interface. This
can be verified by comparing the packet's IP source
address to the interface's IP address, after masking
both addresses with the interface mask. This comparison
should not be performed on point-to-point networks. On
point-to-point networks, the interface addresses of each
end of the link are assigned independently, if they are
assigned at all.
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(2) Indicate the backbone. In this case, the packet has
been sent over a virtual link. The receiving router
must be an area border router, and the Router ID
specified in the packet (the source router) must be the
other end of a configured virtual link. The receiving
interface must also attach to the virtual link's
configured Transit area. If all of these checks
succeed, the packet is accepted and is from now on
associated with the virtual link (and the backbone
area).
o Packets whose IP destination is AllDRouters should only be
accepted if the state of the receiving interface is DR or
Backup (see Section 9.1).
o The AuType specified in the packet must match the AuType
specified for the associated area.
o The packet must be authenticated. The authentication
procedure is indicated by the setting of AuType (see
Appendix D). The authentication procedure may use one or
more Authentication keys, which can be configured on a per-
interface basis. The authentication procedure may also
verify the checksum field in the OSPF packet header (which,
when used, is set to the standard IP 16-bit one's complement
checksum of the OSPF packet's contents after excluding the
64-bit authentication field). If the authentication
procedure fails, the packet should be discarded.
If the packet type is Hello, it should then be further processed by
the Hello Protocol (see Section 10.5). All other packet types are
sent/received only on adjacencies. This means that the packet must
have been sent by one of the router's active neighbors. If the
receiving interface connects to a broadcast network, Point-to-
MultiPoint network or NBMA network the sender is identified by the IP
source address found in the packet's IP header. If the receiving
interface connects to a point-to-point network or a virtual link, the
sender is identified by the Router ID (source router) found in the
packet's OSPF header. The data structure associated with the
receiving interface contains the list of active neighbors. Packets
not matching any active neighbor are discarded.
At this point all received protocol packets are associated with an
active neighbor. For the further input processing of specific packet
types, consult the sections listed in Table 11.
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Type Packet name detailed section (receive)
________________________________________________________
1 Hello Section 10.5
2 Database description Section 10.6
3 Link state request Section 10.7
4 Link state update Section 13
5 Link state ack Section 13.7
Table 11: Sections describing OSPF protocol packet reception.
9. The Interface Data Structure
An OSPF interface is the connection between a router and a network.
We assume a single OSPF interface to each attached network/subnet,
although supporting multiple interfaces on a single network is
considered in Appendix F. Each interface structure has at most one IP
interface address.
An OSPF interface can be considered to belong to the area that
contains the attached network. All routing protocol packets
originated by the router over this interface are labelled with the
interface's Area ID. One or more router adjacencies may develop over
an interface. A router's LSAs reflect the state of its interfaces and
their associated adjacencies.
The following data items are associated with an interface. Note that
a number of these items are actually configuration for the attached
network; such items must be the same for all routers connected to the
network.
Type
The OSPF interface type is either point-to-point, broadcast, NBMA,
Point-to-MultiPoint or virtual link.
State
The functional level of an interface. State determines whether or
not full adjacencies are allowed to form over the interface.
State is also reflected in the router's LSAs.
IP interface address
The IP address associated with the interface. This appears as the
IP source address in all routing protocol packets originated over
this interface. Interfaces to unnumbered point-to-point networks
do not have an associated IP address.
IP interface mask
Also referred to as the subnet mask, this indicates the portion of
the IP interface address that identifies the attached network.
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Masking the IP interface address with the IP interface mask yields
the IP network number of the attached network. On point-to-point
networks and virtual links, the IP interface mask is not defined.
On these networks, the link itself is not assigned an IP network
number, and so the addresses of each side of the link are assigned
independently, if they are assigned at all.
Area ID
The Area ID of the area to which the attached network belongs.
All routing protocol packets originating from the interface are
labelled with this Area ID.
HelloInterval
The length of time, in seconds, between the Hello packets that the
router sends on the interface. Advertised in Hello packets sent
out this interface.
RouterDeadInterval
The number of seconds before the router's neighbors will declare
it down, when they stop hearing the router's Hello Packets.
Advertised in Hello packets sent out this interface.
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update Packet over this interface. LSAs contained in the Link
State Update packet will have their age incremented by this amount
before transmission. This value should take into account
transmission and propagation delays; it must be greater than zero.
Router Priority
An 8-bit unsigned integer. When two routers attached to a network
both attempt to become Designated Router, the one with the highest
Router Priority takes precedence. A router whose Router Priority
is set to 0 is ineligible to become Designated Router on the
attached network. Advertised in Hello packets sent out this
interface.
Hello Timer
An interval timer that causes the interface to send a Hello
packet. This timer fires every HelloInterval seconds. Note that
on non-broadcast networks a separate Hello packet is sent to each
qualified neighbor.
Wait Timer
A single shot timer that causes the interface to exit the Waiting
state, and as a consequence select a Designated Router on the
network. The length of the timer is RouterDeadInterval seconds.
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RFC 2178 OSPF Version 2 July 1997
List of neighboring routers
The other routers attached to this network. This list is formed
by the Hello Protocol. Adjacencies will be formed to some of
these neighbors. The set of adjacent neighbors can be determined
by an examination of all of the neighbors' states.
Designated Router
The Designated Router selected for the attached network. The
Designated Router is selected on all broadcast and NBMA networks
by the Hello Protocol. Two pieces of identification are kept for
the Designated Router: its Router ID and its IP interface address
on the network. The Designated Router advertises link state for
the network; this network-LSA is labelled with the Designated
Router's IP address. The Designated Router is initialized to
0.0.0.0, which indicates the lack of a Designated Router.
Backup Designated Router
The Backup Designated Router is also selected on all broadcast and
NBMA networks by the Hello Protocol. All routers on the attached
network become adjacent to both the Designated Router and the
Backup Designated Router. The Backup Designated Router becomes
Designated Router when the current Designated Router fails. The
Backup Designated Router is initialized to 0.0.0.0, indicating the
lack of a Backup Designated Router.
Interface output cost(s)
The cost of sending a data packet on the interface, expressed in
the link state metric. This is advertised as the link cost for
this interface in the router-LSA. The cost of an interface must be
greater than zero.
RxmtInterval
The number of seconds between LSA retransmissions, for adjacencies
belonging to this interface. Also used when retransmitting
Database Description and Link State Request Packets.
AuType
The type of authentication used on the attached network/subnet.
Authentication types are defined in Appendix D. All OSPF packet
exchanges are authenticated. Different authentication schemes may
be used on different networks/subnets.
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Authentication key
This configured data allows the authentication procedure to
generate and/or verify OSPF protocol packets. The Authentication
key can be configured on a per-interface basis. For example, if
the AuType indicates simple password, the Authentication key would
be a 64-bit clear password which is inserted into the OSPF packet
header. If instead Autype indicates Cryptographic authentication,
then the Authentication key is a shared secret which enables the
generation/verification of message digests which are appended to
the OSPF protocol packets. When Cryptographic authentication is
used, multiple simultaneous keys are supported in order to achieve
smooth key transition (see Section D.3).
9.1. Interface states
The various states that router interfaces may attain is documented in
this section. The states are listed in order of progressing
functionality. For example, the inoperative state is listed first,
followed by a list of intermediate states before the final, fully
functional state is achieved. The specification makes use of this
ordering by sometimes making references such as "those interfaces in
state greater than X". Figure 11 shows the graph of interface state
changes. The arcs of the graph are labelled with the event causing
the state change. These events are documented in Section 9.2. The
interface state machine is described in more detail in Section 9.3.
Down
This is the initial interface state. In this state, the lower-
level protocols have indicated that the interface is unusable. No
protocol traffic at all will be sent or received on such a
interface. In this state, interface parameters should be set to
their initial values.
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+----+ UnloopInd +--------+
|Down|<--------------|Loopback|
+----+ +--------+
|
|InterfaceUp
+-------+ | +--------------+
|Waiting|<-+-------------->|Point-to-point|
+-------+ +--------------+
|
WaitTimer|BackupSeen
|
|
| NeighborChange
+------+ +-+<---------------- +-------+
|Backup|<----------|?|----------------->|DROther|
+------+---------->+-+<-----+ +-------+
Neighbor | |
Change | |Neighbor
| |Change
| +--+
+---->|DR|
+--+
Figure 11: Interface State changes
In addition to the state transitions pictured,
Event InterfaceDown always forces Down State, and
Event LoopInd always forces Loopback State
All interface timers should be disabled, and there should be no
adjacencies associated with the interface.
Loopback
In this state, the router's interface to the network is looped
back. The interface may be looped back in hardware or software.
The interface will be unavailable for regular data traffic.
However, it may still be desirable to gain information on the
quality of this interface, either through sending ICMP pings to
the interface or through something like a bit error test. For
this reason, IP packets may still be addressed to an interface in
Loopback state. To facilitate this, such interfaces are
advertised in router-LSAs as single host routes, whose destination
is the IP interface address.[4]
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Waiting
In this state, the router is trying to determine the identity of
the (Backup) Designated Router for the network. To do this, the
router monitors the Hello Packets it receives. The router is not
allowed to elect a Backup Designated Router nor a Designated
Router until it transitions out of Waiting state. This prevents
unnecessary changes of (Backup) Designated Router.
Point-to-point
In this state, the interface is operational, and connects either
to a physical point-to-point network or to a virtual link. Upon
entering this state, the router attempts to form an adjacency with
the neighboring router. Hello Packets are sent to the neighbor
every HelloInterval seconds.
DR Other
The interface is to a broadcast or NBMA network on which another
router has been selected to be the Designated Router. In this
state, the router itself has not been selected Backup Designated
Router either. The router forms adjacencies to both the
Designated Router and the Backup Designated Router (if they
exist).
Backup
In this state, the router itself is the Backup Designated Router
on the attached network. It will be promoted to Designated Router
when the present Designated Router fails. The router establishes
adjacencies to all other routers attached to the network. The
Backup Designated Router performs slightly different functions
during the Flooding Procedure, as compared to the Designated
Router (see Section 13.3). See Section 7.4 for more details on
the functions performed by the Backup Designated Router.
DR In this state, this router itself is the Designated Router
on the attached network. Adjacencies are established to all other
routers attached to the network. The router must also originate a
network-LSA for the network node. The network-LSA will contain
links to all routers (including the Designated Router itself)
attached to the network. See Section 7.3 for more details on the
functions performed by the Designated Router.
9.2. Events causing interface state changes
State changes can be effected by a number of events. These events
are pictured as the labelled arcs in Figure 11. The label
definitions are listed below. For a detailed explanation of the
effect of these events on OSPF protocol operation, consult Section9.3.
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InterfaceUp
Lower-level protocols have indicated that the network interface is
operational. This enables the interface to transition out of Down
state. On virtual links, the interface operational indication is
actually a result of the shortest path calculation (see Section16.7).
WaitTimer
The Wait Timer has fired, indicating the end of the waiting period
that is required before electing a (Backup) Designated Router.
BackupSeen
The router has detected the existence or non-existence of a Backup
Designated Router for the network. This is done in one of two
ways. First, an Hello Packet may be received from a neighbor
claiming to be itself the Backup Designated Router.
Alternatively, an Hello Packet may be received from a neighbor
claiming to be itself the Designated Router, and indicating that
there is no Backup Designated Router. In either case there must
be bidirectional communication with the neighbor, i.e., the router
must also appear in the neighbor's Hello Packet. This event
signals an end to the Waiting state.
NeighborChange
There has been a change in the set of bidirectional neighbors
associated with the interface. The (Backup) Designated Router
needs to be recalculated. The following neighbor changes lead to
the NeighborChange event. For an explanation of neighbor states,
see Section 10.1.
o Bidirectional communication has been established to a
neighbor. In other words, the state of the neighbor has
transitioned to 2-Way or higher.
o There is no longer bidirectional communication with a
neighbor. In other words, the state of the neighbor has
transitioned to Init or lower.
o One of the bidirectional neighbors is newly declaring
itself as either Designated Router or Backup Designated
Router. This is detected through examination of that
neighbor's Hello Packets.
o One of the bidirectional neighbors is no longer
declaring itself as Designated Router, or is no longer
declaring itself as Backup Designated Router. This is
again detected through examination of that neighbor's
Hello Packets.
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o The advertised Router Priority for a bidirectional
neighbor has changed. This is again detected through
examination of that neighbor's Hello Packets.
LoopInd
An indication has been received that the interface is now looped
back to itself. This indication can be received either from
network management or from the lower level protocols.
UnloopInd
An indication has been received that the interface is no longer
looped back. As with the LoopInd event, this indication can be
received either from network management or from the lower level
protocols.
InterfaceDown
Lower-level protocols indicate that this interface is no longer
functional. No matter what the current interface state is, the new
interface state will be Down.
9.3. The Interface state machine
A detailed description of the interface state changes follows. Each
state change is invoked by an event (Section 9.2). This event may
produce different effects, depending on the current state of the
interface. For this reason, the state machine below is organized by
current interface state and received event. Each entry in the state
machine describes the resulting new interface state and the required
set of additional actions.
When an interface's state changes, it may be necessary to originate a
new router-LSA. See Section 12.4 for more details.
Some of the required actions below involve generating events for the
neighbor state machine. For example, when an interface becomes
inoperative, all neighbor connections associated with the interface
must be destroyed. For more information on the neighbor state
machine, see Section 10.3.
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State(s): Down
Event: InterfaceUp
New state: Depends upon action routine
Action: Start the interval Hello Timer, enabling the
periodic sending of Hello packets out the interface.
If the attached network is a physical point-to-point
network, Point-to-MultiPoint network or virtual
link, the interface state transitions to Point-to-
Point. Else, if the router is not eligible to
become Designated Router the interface state
transitions to DR Other.
Otherwise, the attached network is a broadcast or
NBMA network and the router is eligible to become
Designated Router. In this case, in an attempt to
discover the attached network's Designated Router
the interface state is set to Waiting and the single
shot Wait Timer is started. Additionally, if the
network is an NBMA network examine the configured
list of neighbors for this interface and generate
the neighbor event Start for each neighbor that is
also eligible to become Designated Router.
State(s): Waiting
Event: BackupSeen
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Waiting
Event: WaitTimer
New state: Depends upon action routine.
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Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): DR Other, Backup or DR
Event: NeighborChange
New state: Depends upon action routine.
Action: Recalculate the attached network's Backup Designated
Router and Designated Router, as shown in Section9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Any State
Event: InterfaceDown
New state: Down
Action: All interface variables are reset, and interface
timers disabled. Also, all neighbor connections
associated with the interface are destroyed. This
is done by generating the event KillNbr on all
associated neighbors (see Section 10.2).
State(s): Any State
Event: LoopInd
New state: Loopback
Action: Since this interface is no longer connected to the
attached network the actions associated with the
above InterfaceDown event are executed.
State(s): Loopback
Event: UnloopInd
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New state: Down
Action: No actions are necessary. For example, the
interface variables have already been reset upon
entering the Loopback state. Note that reception of
an InterfaceUp event is necessary before the
interface again becomes fully functional.
9.4. Electing the Designated Router
This section describes the algorithm used for calculating a network's
Designated Router and Backup Designated Router. This algorithm is
invoked by the Interface state machine. The initial time a router
runs the election algorithm for a network, the network's Designated
Router and Backup Designated Router are initialized to 0.0.0.0. This
indicates the lack of both a Designated Router and a Backup
Designated Router.
The Designated Router election algorithm proceeds as follows: Call
the router doing the calculation Router X. The list of neighbors
attached to the network and having established bidirectional
communication with Router X is examined. This list is precisely the
collection of Router X's neighbors (on this network) whose state is
greater than or equal to 2-Way (see Section 10.1). Router X itself
is also considered to be on the list. Discard all routers from the
list that are ineligible to become Designated Router. (Routers
having Router Priority of 0 are ineligible to become Designated
Router.) The following steps are then executed, considering only
those routers that remain on the list:
(1) Note the current values for the network's Designated Router
and Backup Designated Router. This is used later for
comparison purposes.
(2) Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
declared themselves to be Designated Router are eligible to
become Backup Designated Router. If one or more of these
routers have declared themselves Backup Designated Router
(i.e., they are currently listing themselves as Backup
Designated Router, but not as Designated Router, in their
Hello Packets) the one having highest Router Priority is
declared to be Backup Designated Router. In case of a tie,
the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router,
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choose the router having highest Router Priority, (again
excluding those routers who have declared themselves
Designated Router), and again use the Router ID to break
ties.
(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
themselves Designated Router (i.e., they are currently
listing themselves as Designated Router in their Hello
Packets) the one having highest Router Priority is declared
to be Designated Router. In case of a tie, the one having
the highest Router ID is chosen. If no routers have
declared themselves Designated Router, assign the Designated
Router to be the same as the newly elected Backup Designated
Router.
(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated
Router or no longer the Backup Designated Router, repeat
steps 2 and 3, and then proceed to step 5. For example, if
Router X is now the Designated Router, when step 2 is
repeated X will no longer be eligible for Backup Designated
Router election. Among other things, this will ensure that
no router will declare itself both Backup Designated Router
and Designated Router.[5]
(5) As a result of these calculations, the router itself may now
be Designated Router or Backup Designated Router. See
Sections 7.3 and 7.4 for the additional duties this would
entail. The router's interface state should be set
accordingly. If the router itself is now Designated Router,
the new interface state is DR. If the router itself is now
Backup Designated Router, the new interface state is Backup.
Otherwise, the new interface state is DR Other.
(6) If the attached network is an NBMA network, and the router
itself has just become either Designated Router or Backup
Designated Router, it must start sending Hello Packets to
those neighbors that are not eligible to become Designated
Router (see Section 9.5.1). This is done by invoking the
neighbor event Start for each neighbor having a Router
Priority of 0.
(7) If the above calculations have caused the identity of either
the Designated Router or Backup Designated Router to change,
the set of adjacencies associated with this interface will
need to be modified. Some adjacencies may need to be
formed, and others may need to be broken. To accomplish
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this, invoke the event AdjOK? on all neighbors whose state
is at least 2-Way. This will cause their eligibility for
adjacency to be reexamined (see Sections 10.3 and 10.4).
The reason behind the election algorithm's complexity is the desire
for an orderly transition from Backup Designated Router to Designated
Router, when the current Designated Router fails. This orderly
transition is ensured through the introduction of hysteresis: no new
Backup Designated Router can be chosen until the old Backup accepts
its new Designated Router responsibilities.
The above procedure may elect the same router to be both Designated
Router and Backup Designated Router, although that router will never
be the calculating router (Router X) itself. The elected Designated
Router may not be the router having the highest Router Priority, nor
will the Backup Designated Router necessarily have the second highest
Router Priority. If Router X is not itself eligible to become
Designated Router, it is possible that neither a Backup Designated
Router nor a Designated Router will be selected in the above
procedure. Note also that if Router X is the only attached router
that is eligible to become Designated Router, it will select itself
as Designated Router and there will be no Backup Designated Router
for the network.
9.5. Sending Hello packets
Hello packets are sent out each functioning router interface. They
are used to discover and maintain neighbor relationships.[6] On
broadcast and NBMA networks, Hello Packets are also used to elect the
Designated Router and Backup Designated Router.
The format of an Hello packet is detailed in Section A.3.2. The
Hello Packet contains the router's Router Priority (used in choosing
the Designated Router), and the interval between Hello Packets sent
out the interface (HelloInterval). The Hello Packet also indicates
how often a neighbor must be heard from to remain active
(RouterDeadInterval). Both HelloInterval and RouterDeadInterval must
be the same for all routers attached to a common network. The Hello
packet also contains the IP address mask of the attached network
(Network Mask). On unnumbered point-to-point networks and on virtual
links this field should be set to 0.0.0.0.
The Hello packet's Options field describes the router's optional OSPF
capabilities. One optional capability is defined in this
specification (see Sections 4.5 and A.2). The E-bit of the Options
field should be set if and only if the attached area is capable of
processing AS-external-LSAs (i.e., it is not a stub area). If the E-
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bit is set incorrectly the neighboring routers will refuse to accept
the Hello Packet (see Section 10.5). Unrecognized bits in the Hello
Packet's Options field should be set to zero.
In order to ensure two-way communication between adjacent routers,
the Hello packet contains the list of all routers on the network from
which Hello Packets have been seen recently. The Hello packet also
contains the router's current choice for Designated Router and Backup
Designated Router. A value of 0.0.0.0 in these fields means that one
has not yet been selected.
On broadcast networks and physical point-to-point networks, Hello
packets are sent every HelloInterval seconds to the IP multicast
address AllSPFRouters. On virtual links, Hello packets are sent as
unicasts (addressed directly to the other end of the virtual link)
every HelloInterval seconds. On Point-to-MultiPoint networks,
separate Hello packets are sent to each attached neighbor every
HelloInterval seconds. Sending of Hello packets on NBMA networks is
covered in the next section.
9.5.1. Sending Hello packets on NBMA networks
Static configuration information may be necessary in order for the
Hello Protocol to function on non-broadcast networks (see Sections
C.5 and C.6). On NBMA networks, every attached router which is
eligible to become Designated Router becomes aware of all of its
neighbors on the network (either through configuration or by some
unspecified mechanism). Each neighbor is labelled with the
neighbor's Designated Router eligibility.
The interface state must be at least Waiting for any Hello Packets to
be sent out the NBMA interface. Hello Packets are then sent directly
(as unicasts) to some subset of a router's neighbors. Sometimes an
Hello Packet is sent periodically on a timer; at other times it is
sent as a response to a received Hello Packet. A router's hello-
sending behavior varies depending on whether the router itself is
eligible to become Designated Router.
If the router is eligible to become Designated Router, it must
periodically send Hello Packets to all neighbors that are also
eligible. In addition, if the router is itself the Designated Router
or Backup Designated Router, it must also send periodic Hello Packets
to all other neighbors. This means that any two eligible routers are
always exchanging Hello Packets, which is necessary for the correct
operation of the Designated Router election algorithm. To minimize
the number of Hello Packets sent, the number of eligible routers on
an NBMA network should be kept small.
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If the router is not eligible to become Designated Router, it must
periodically send Hello Packets to both the Designated Router and the
Backup Designated Router (if they exist). It must also send an Hello
Packet in reply to an Hello Packet received from any eligible
neighbor (other than the current Designated Router and Backup
Designated Router). This is needed to establish an initial
bidirectional relationship with any potential Designated Router.
When sending Hello packets periodically to any neighbor, the interval
between Hello Packets is determined by the neighbor's state. If the
neighbor is in state Down, Hello Packets are sent every PollInterval
seconds. Otherwise, Hello Packets are sent every HelloInterval
seconds.
10. The Neighbor Data Structure
An OSPF router converses with its neighboring routers. Each separate
conversation is described by a "neighbor data structure". Each
conversation is bound to a particular OSPF router interface, and is
identified either by the neighboring router's OSPF Router ID or by
its Neighbor IP address (see below). Thus if the OSPF router and
another router have multiple attached networks in common, multiple
conversations ensue, each described by a unique neighbor data
structure. Each separate conversation is loosely referred to in the
text as being a separate "neighbor".
The neighbor data structure contains all information pertinent to the
forming or formed adjacency between the two neighbors. (However,
remember that not all neighbors become adjacent.) An adjacency can
be viewed as a highly developed conversation between two routers.
State
The functional level of the neighbor conversation. This is
described in more detail in Section 10.1.
Inactivity Timer
A single shot timer whose firing indicates that no Hello Packet
has been seen from this neighbor recently. The length of the
timer is RouterDeadInterval seconds.
Master/Slave
When the two neighbors are exchanging databases, they form a
master/slave relationship. The master sends the first Database
Description Packet, and is the only part that is allowed to
retransmit. The slave can only respond to the master's Database
Description Packets. The master/slave relationship is negotiated
in state ExStart.
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DD Sequence Number
The DD Sequence number of the Database Description packet that is
currently being sent to the neighbor.
Last received Database Description packet
The initialize(I), more (M) and master(MS) bits, Options field,
and DD sequence number contained in the last Database Description
packet received from the neighbor. Used to determine whether the
next Database Description packet received from the neighbor is a
duplicate.
Neighbor ID
The OSPF Router ID of the neighboring router. The Neighbor ID is
learned when Hello packets are received from the neighbor, or is
configured if this is a virtual adjacency (see Section C.4).
Neighbor Priority
The Router Priority of the neighboring router. Contained in the
neighbor's Hello packets, this item is used when selecting the
Designated Router for the attached network.
Neighbor IP address
The IP address of the neighboring router's interface to the
attached network. Used as the Destination IP address when
protocol packets are sent as unicasts along this adjacency. Also
used in router-LSAs as the Link ID for the attached network if the
neighboring router is selected to be Designated Router (see
Section 12.4.1). The Neighbor IP address is learned when Hello
packets are received from the neighbor. For virtual links, the
Neighbor IP address is learned during the routing table build
process (see Section 15).
Neighbor Options
The optional OSPF capabilities supported by the neighbor. Learned
during the Database Exchange process (see Section 10.6). The
neighbor's optional OSPF capabilities are also listed in its Hello
packets. This enables received Hello Packets to be rejected (i.e.,
neighbor relationships will not even start to form) if there is a
mismatch in certain crucial OSPF capabilities (see Section 10.5).
The optional OSPF capabilities are documented in Section 4.5.
Neighbor's Designated Router
The neighbor's idea of the Designated Router. If this is the
neighbor itself, this is important in the local calculation of the
Designated Router. Defined only on broadcast and NBMA networks.
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Neighbor's Backup Designated Router
The neighbor's idea of the Backup Designated Router. If this is
the neighbor itself, this is important in the local calculation of
the Backup Designated Router. Defined only on broadcast and NBMA
networks.
The next set of variables are lists of LSAs. These lists describe
subsets of the area link-state database. This memo defines five
distinct types of LSAs, all of which may be present in an area link-
state database: router-LSAs, network-LSAs, and Type 3 and 4 summary-
LSAs (all stored in the area data structure), and AS- external-LSAs
(stored in the global data structure).
Link state retransmission list
The list of LSAs that have been flooded but not acknowledged on
this adjacency. These will be retransmitted at intervals until
they are acknowledged, or until the adjacency is destroyed.
Database summary list
The complete list of LSAs that make up the area link-state
database, at the moment the neighbor goes into Database Exchange
state. This list is sent to the neighbor in Database Description
packets.
Link state request list
The list of LSAs that need to be received from this neighbor in
order to synchronize the two neighbors' link-state databases.
This list is created as Database Description packets are received,
and is then sent to the neighbor in Link State Request packets.
The list is depleted as appropriate Link State Update packets are
received.
10.1. Neighbor states
The state of a neighbor (really, the state of a conversation being
held with a neighboring router) is documented in the following
sections. The states are listed in order of progressing
functionality. For example, the inoperative state is listed first,
followed by a list of intermediate states before the final, fully
functional state is achieved. The specification makes use of this
ordering by sometimes making references such as "those
neighbors/adjacencies in state greater than X". Figures 12 and 13
show the graph of neighbor state changes. The arcs of the graphs are
labelled with the event causing the state change. The neighbor
events are documented in Section 10.2.
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The graph in Figure 12 shows the state changes effected by the Hello
Protocol. The Hello Protocol is responsible for neighbor acquisition
and maintenance, and for ensuring two way communication between
neighbors.
The graph in Figure 13 shows the forming of an adjacency. Not every
two neighboring routers become adjacent (see Section 10.4). The
adjacency starts to form when the neighbor is in state ExStart.
After the two routers discover their master/slave status, the state
transitions to Exchange. At this point the neighbor starts to be
used in the flooding procedure, and the two neighboring routers begin
synchronizing their databases. When this synchronization is
finished, the neighbor is in state Full and we say that the two
routers are fully adjacent. At this point the adjacency is listed in
LSAs.
For a more detailed description of neighbor state changes, together
with the additional actions involved in each change, see Section10.3.
Down
This is the initial state of a neighbor conversation. It
indicates that there has been no recent information received from
the neighbor. On NBMA networks, Hello packets may still be sent to
"Down" neighbors, although at a reduced frequency (see Section9.5.1).
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Attempt
This state is only valid for neighbors attached to NBMA networks.
It indicates that no recent information has been received from the
neighbor, but that a more concerted effort should be made to
contact the neighbor. This is done by sending the neighbor Hello
packets at intervals of HelloInterval (see Section 9.5.1).
Init
In this state, an Hello packet has recently been seen from the
neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). All neighbors in this
state (or higher) are listed in the Hello packets sent from the
associated interface.
2-Way
In this state, communication between the two routers is
bidirectional. This has been assured by the operation of the
Hello Protocol. This is the most advanced state short of
beginning adjacency establishment. The (Backup) Designated Router
is selected from the set of neighbors in state 2-Way or greater.
ExStart
This is the first step in creating an adjacency between the two
neighboring routers. The goal of this step is to decide which
router is the master, and to decide upon the initial DD sequence
number. Neighbor conversations in this state or greater are
called adjacencies.
Exchange
In this state the router is describing its entire link state
database by sending Database Description packets to the neighbor.
Each Database Description Packet has a DD sequence number, and is
explicitly acknowledged. Only one Database Description Packet is
allowed outstanding at any one time. In this state, Link State
Request Packets may also be sent asking for the neighbor's more
recent LSAs. All adjacencies in Exchange state or greater are
used by the flooding procedure. In fact, these adjacencies are
fully capable of transmitting and receiving all types of OSPF
routing protocol packets.
Loading
In this state, Link State Request packets are sent to the neighbor
asking for the more recent LSAs that have been discovered (but not
yet received) in the Exchange state.
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Full
In this state, the neighboring routers are fully adjacent. These
adjacencies will now appear in router-LSAs and network-LSAs.
10.2. Events causing neighbor state changes
State changes can be effected by a number of events. These events
are shown in the labels of the arcs in Figures 12 and 13. The label
definitions are as follows:
HelloReceived
An Hello packet has been received from the neighbor.
Start
This is an indication that Hello Packets should now be sent to the
neighbor at intervals of HelloInterval seconds. This event is
generated only for neighbors associated with NBMA networks.
2-WayReceived
Bidirectional communication has been realized between the two
neighboring routers. This is indicated by the router seeing
itself in the neighbor's Hello packet.
NegotiationDone
The Master/Slave relationship has been negotiated, and DD sequence
numbers have been exchanged. This signals the start of the
sending/receiving of Database Description packets. For more
information on the generation of this event, consult Section 10.8.
ExchangeDone
Both routers have successfully transmitted a full sequence of
Database Description packets. Each router now knows what parts of
its link state database are out of date. For more information on
the generation of this event, consult Section 10.8.
BadLSReq
A Link State Request has been received for an LSA not contained in
the database. This indicates an error in the Database Exchange
process.
Loading Done
Link State Updates have been received for all out-of-date portions
of the database. This is indicated by the Link state request list
becoming empty after the Database Exchange process has completed.
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AdjOK?
A decision must be made as to whether an adjacency should be
established/maintained with the neighbor. This event will start
some adjacencies forming, and destroy others.
The following events cause well developed neighbors to revert to
lesser states. Unlike the above events, these events may occur when
the neighbor conversation is in any of a number of states.
SeqNumberMismatch
A Database Description packet has been received that either a) has
an unexpected DD sequence number, b) unexpectedly has the Init bit
set or c) has an Options field differing from the last Options
field received in a Database Description packet. Any of these
conditions indicate that some error has occurred during adjacency
establishment.
1-Way
An Hello packet has been received from the neighbor, in which the
router is not mentioned. This indicates that communication with
the neighbor is not bidirectional.
KillNbr
This is an indication that all communication with the neighbor is
now impossible, forcing the neighbor to revert to Down state.
InactivityTimer
The inactivity Timer has fired. This means that no Hello packets
have been seen recently from the neighbor. The neighbor reverts to
Down state.
LLDown
This is an indication from the lower level protocols that the
neighbor is now unreachable. For example, on an X.25 network this
could be indicated by an X.25 clear indication with appropriate
cause and diagnostic fields. This event forces the neighbor into
Down state.
10.3. The Neighbor state machine
A detailed description of the neighbor state changes follows. Each
state change is invoked by an event (Section 10.2). This event may
produce different effects, depending on the current state of the
neighbor. For this reason, the state machine below is organized by
current neighbor state and received event. Each entry in the state
machine describes the resulting new neighbor state and the required
set of additional actions.
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When a neighbor's state changes, it may be necessary to rerun the
Designated Router election algorithm. This is determined by whether
the interface NeighborChange event is generated (see Section 9.2).
Also, if the Interface is in DR state (the router is itself
Designated Router), changes in neighbor state may cause a new
network-LSA to be originated (see Section 12.4).
When the neighbor state machine needs to invoke the interface state
machine, it should be done as a scheduled task (see Section 4.4).
This simplifies things, by ensuring that neither state machine will
be executed recursively.
State(s): Down
Event: Start
New state: Attempt
Action: Send an Hello Packet to the neighbor (this neighbor
is always associated with an NBMA network) and start
the Inactivity Timer for the neighbor. The timer's
later firing would indicate that communication with
the neighbor was not attained.
State(s): Attempt
Event: HelloReceived
New state: Init
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has now been heard from.
State(s): Down
Event: HelloReceived
New state: Init
Action: Start the Inactivity Timer for the neighbor. The
timer's later firing would indicate that the neighbor
is dead.
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State(s): Init or greater
Event: HelloReceived
New state: No state change.
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has again been heard from.
State(s): Init
Event: 2-WayReceived
New state: Depends upon action routine.
Action: Determine whether an adjacency should be established
with the neighbor (see Section 10.4). If not, the
new neighbor state is 2-Way.
Otherwise (an adjacency should be established) the
neighbor state transitions to ExStart. Upon
entering this state, the router increments the DD
sequence number in the neighbor data structure. If
this is the first time that an adjacency has been
attempted, the DD sequence number should be assigned
some unique value (like the time of day clock). It
then declares itself master (sets the master/slave
bit to master), and starts sending Database
Description Packets, with the initialize (I), more
(M) and master (MS) bits set. This Database
Description Packet should be otherwise empty. This
Database Description Packet should be retransmitted
at intervals of RxmtInterval until the next state is
entered (see Section 10.8).
State(s): ExStart
Event: NegotiationDone
New state: Exchange
Action: The router must list the contents of its entire area
link state database in the neighbor Database summary
list. The area link state database consists of the
router-LSAs, network-LSAs and summary-LSAs contained
in the area structure, along with the AS-external-
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LSAs contained in the global structure. AS-
external-LSAs are omitted from a virtual neighbor's
Database summary list. AS-external-LSAs are omitted
from the Database summary list if the area has been
configured as a stub (see Section 3.6). LSAs whose
age is equal to MaxAge are instead added to the
neighbor's Link state retransmission list. A
summary of the Database summary list will be sent to
the neighbor in Database Description packets. Each
Database Description Packet has a DD sequence
number, and is explicitly acknowledged. Only one
Database Description Packet is allowed outstanding
at any one time. For more detail on the sending and
receiving of Database Description packets, see
Sections 10.8 and 10.6.
State(s): Exchange
Event: ExchangeDone
New state: Depends upon action routine.
Action: If the neighbor Link state request list is empty,
the new neighbor state is Full. No other action is
required. This is an adjacency's final state.
Otherwise, the new neighbor state is Loading. Start
(or continue) sending Link State Request packets to
the neighbor (see Section 10.9). These are requests
for the neighbor's more recent LSAs (which were
discovered but not yet received in the Exchange
state). These LSAs are listed in the Link state
request list associated with the neighbor.
State(s): Loading
Event: Loading Done
New state: Full
Action: No action required. This is an adjacency's final
state.
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State(s): 2-Way
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether an adjacency should be formed with
the neighboring router (see Section 10.4). If not,
the neighbor state remains at 2-Way. Otherwise,
transition the neighbor state to ExStart and perform
the actions associated with the above state machine
entry for state Init and event 2-WayReceived.
State(s): ExStart or greater
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether the neighboring router should
still be adjacent. If yes, there is no state change
and no further action is necessary.
Otherwise, the (possibly partially formed) adjacency
must be destroyed. The neighbor state transitions
to 2-Way. The Link state retransmission list,
Database summary list and Link state request list
are cleared of LSAs.
State(s): Exchange or greater
Event: SeqNumberMismatch
New state: ExStart
Action: The (possibly partially formed) adjacency is torn
down, and then an attempt is made at
reestablishment. The neighbor state first
transitions to ExStart. The Link state
retransmission list, Database summary list and Link
state request list are cleared of LSAs. Then the
router increments the DD sequence number in the
neighbor data structure, declares itself master
(sets the master/slave bit to master), and starts
sending Database Description Packets, with the
initialize (I), more (M) and master (MS) bits set.
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This Database Description Packet should be otherwise
empty (see Section 10.8).
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action: The action for event BadLSReq is exactly the same as
for the neighbor event SeqNumberMismatch. The
(possibly partially formed) adjacency is torn down,
and then an attempt is made at reestablishment. For
more information, see the neighbor state machine
entry that is invoked when event SeqNumberMismatch
is generated in state Exchange or greater.
State(s): Any state
Event: KillNbr
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs. Also, the Inactivity Timer is disabled.
State(s): Any state
Event: LLDown
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs. Also, the Inactivity Timer is disabled.
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State(s): Any state
Event: InactivityTimer
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs.
State(s): 2-Way or greater
Event: 1-WayReceived
New state: Init
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs.
State(s): 2-Way or greater
Event: 2-WayReceived
New state: No state change.
Action: No action required.
State(s): Init
Event: 1-WayReceived
New state: No state change.
Action: No action required.
10.4. Whether to become adjacent
Adjacencies are established with some subset of the router's
neighbors. Routers connected by point-to-point networks, Point-to-
MultiPoint networks and virtual links always become adjacent. On
broadcast and NBMA networks, all routers become adjacent to both the
Designated Router and the Backup Designated Router.
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The adjacency-forming decision occurs in two places in the neighbor
state machine. First, when bidirectional communication is initially
established with the neighbor, and secondly, when the identity of the
attached network's (Backup) Designated Router changes. If the
decision is made to not attempt an adjacency, the state of the
neighbor communication stops at 2-Way.
An adjacency should be established with a bidirectional neighbor when
at least one of the following conditions holds:
o The underlying network type is point-to-point
o The underlying network type is Point-to-MultiPoint
o The underlying network type is virtual link
o The router itself is the Designated Router
o The router itself is the Backup Designated Router
o The neighboring router is the Designated Router
o The neighboring router is the Backup Designated Router
10.5. Receiving Hello Packets
This section explains the detailed processing of a received Hello
Packet. (See Section A.3.2 for the format of Hello packets.) The
generic input processing of OSPF packets will have checked the
validity of the IP header and the OSPF packet header. Next, the
values of the Network Mask, HelloInterval, and RouterDeadInterval
fields in the received Hello packet must be checked against the
values configured for the receiving interface. Any mismatch causes
processing to stop and the packet to be dropped. In other words, the
above fields are really describing the attached network's
configuration. However, there is one exception to the above rule: on
point-to-point networks and on virtual links, the Network Mask in the
received Hello Packet should be ignored.
The receiving interface attaches to a single OSPF area (this could be
the backbone). The setting of the E-bit found in the Hello Packet's
Options field must match this area's ExternalRoutingCapability. If
AS-external-LSAs are not flooded into/throughout the area (i.e, the
area is a "stub") the E-bit must be clear in received Hello Packets,
otherwise the E-bit must be set. A mismatch causes processing to
stop and the packet to be dropped. The setting of the rest of the
bits in the Hello Packet's Options field should be ignored.
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At this point, an attempt is made to match the source of the Hello
Packet to one of the receiving interface's neighbors. If the
receiving interface connects to a broadcast, Point-to-MultiPoint or
NBMA network the source is identified by the IP source address found
in the Hello's IP header. If the receiving interface connects to a
point-to-point link or a virtual link, the source is identified by
the Router ID found in the Hello's OSPF packet header. The
interface's current list of neighbors is contained in the interface's
data structure. If a matching neighbor structure cannot be found,
(i.e., this is the first time the neighbor has been detected), one is
created. The initial state of a newly created neighbor is set to
Down.
When receiving an Hello Packet from a neighbor on a broadcast,
Point-to-MultiPoint or NBMA network, set the neighbor structure's
Neighbor ID equal to the Router ID found in the packet's OSPF header.
When receiving an Hello on a point-to-point network (but not on a
virtual link) set the neighbor structure's Neighbor IP address to the
packet's IP source address.
Now the rest of the Hello Packet is examined, generating events to be
given to the neighbor and interface state machines. These state
machines are specified either to be executed or scheduled (see
Section 4.4). For example, by specifying below that the neighbor
state machine be executed in line, several neighbor state transitions
may be effected by a single received Hello:
o Each Hello Packet causes the neighbor state machine to be
executed with the event HelloReceived.
o Then the list of neighbors contained in the Hello Packet is
examined. If the router itself appears in this list, the
neighbor state machine should be executed with the event 2-
WayReceived. Otherwise, the neighbor state machine should
be executed with the event 1-WayReceived, and the processing
of the packet stops.
o Next, the Hello Packet's Router Priority field is examined.
If this field is different than the one previously received
from the neighbor, the receiving interface's state machine
is scheduled with the event NeighborChange. In any case,
the Router Priority field in the neighbor data structure
should be updated accordingly.
o Next the Designated Router field in the Hello Packet is
examined. If the neighbor is both declaring itself to be
Designated Router (Designated Router field = Neighbor IP
address) and the Backup Designated Router field in the
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packet is equal to 0.0.0.0 and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Designated Router and it
had not previously, or the neighbor is not declaring itself
Designated Router where it had previously, the receiving
interface's state machine is scheduled with the event
NeighborChange. In any case, the Neighbors' Designated
Router item in the neighbor structure is updated
accordingly.
o Finally, the Backup Designated Router field in the Hello
Packet is examined. If the neighbor is declaring itself to
be Backup Designated Router (Backup Designated Router field
= Neighbor IP address) and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Backup Designated Router
and it had not previously, or the neighbor is not declaring
itself Backup Designated Router where it had previously, the
receiving interface's state machine is scheduled with the
event NeighborChange. In any case, the Neighbor's Backup
Designated Router item in the neighbor structure is updated
accordingly.
On NBMA networks, receipt of an Hello Packet may also cause an Hello
Packet to be sent back to the neighbor in response. See Section 9.5.1
for more details.
10.6. Receiving Database Description Packets
This section explains the detailed processing of a received Database
Description Packet. The incoming Database Description Packet has
already been associated with a neighbor and receiving interface by
the generic input packet processing (Section 8.2). Whether the
Database Description packet should be accepted, and if so, how it
should be further processed depends upon the neighbor state.
If a Database Description packet is accepted, the following packet
fields should be saved in the corresponding neighbor data structure
under "last received Database Description packet": the packet's
initialize(I), more (M) and master(MS) bits, Options field, and DD
sequence number. If these fields are set identically in two
consecutive Database Description packets received from the neighbor,
the second Database Description packet is considered to be a
"duplicate" in the processing described below.
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If the Interface MTU field in the Database Description packet
indicates an IP datagram size that is larger than the router can
accept on the receiving interface without fragmentation, the Database
Description packet is rejected. Otherwise, if the neighbor state is:
Down
The packet should be rejected.
Attempt
The packet should be rejected.
Init
The neighbor state machine should be executed with the event 2-
WayReceived. This causes an immediate state change to either
state 2-Way or state ExStart. If the new state is ExStart, the
processing of the current packet should then continue in this new
state by falling through to case ExStart below.
2-Way
The packet should be ignored. Database Description Packets are
used only for the purpose of bringing up adjacencies.[7]
ExStart
If the received packet matches one of the following cases, then
the neighbor state machine should be executed with the event
NegotiationDone (causing the state to transition to Exchange), the
packet's Options field should be recorded in the neighbor
structure's Neighbor Options field and the packet should be
accepted as next in sequence and processed further (see below).
Otherwise, the packet should be ignored.
o The initialize(I), more (M) and master(MS) bits are set,
the contents of the packet are empty, and the neighbor's
Router ID is larger than the router's own. In this case
the router is now Slave. Set the master/slave bit to
slave, and set the neighbor data structure's DD sequence
number to that specified by the master.
o The initialize(I) and master(MS) bits are off, the
packet's DD sequence number equals the neighbor data
structure's DD sequence number (indicating
acknowledgment) and the neighbor's Router ID is smaller
than the router's own. In this case the router is
Master.
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Exchange
Duplicate Database Description packets are discarded by the
master, and cause the slave to retransmit the last Database
Description packet that it had sent. Otherwise (the packet is not
a duplicate):
o If the state of the MS-bit is inconsistent with the
master/slave state of the connection, generate the
neighbor event SeqNumberMismatch and stop processing the
packet.
o If the initialize(I) bit is set, generate the neighbor
event SeqNumberMismatch and stop processing the packet.
o If the packet's Options field indicates a different set
of optional OSPF capabilities than were previously
received from the neighbor (recorded in the Neighbor
Options field of the neighbor structure), generate the
neighbor event SeqNumberMismatch and stop processing the
packet.
o Database Description packets must be processed in
sequence, as indicated by the packets' DD sequence
numbers. If the router is master, the next packet
received should have DD sequence number equal to the DD
sequence number in the neighbor data structure. If the
router is slave, the next packet received should have DD
sequence number equal to one more than the DD sequence
number stored in the neighbor data structure. In either
case, if the packet is the next in sequence it should be
accepted and its contents processed as specified below.
o Else, generate the neighbor event SeqNumberMismatch and
stop processing the packet.
Loading or Full
In this state, the router has sent and received an entire sequence
of Database Description Packets. The only packets received should
be duplicates (see above). In particular, the packet's Options
field should match the set of optional OSPF capabilities
previously indicated by the neighbor (stored in the neighbor
structure's Neighbor Options field). Any other packets received,
including the reception of a packet with the Initialize(I) bit
set, should generate the neighbor event SeqNumberMismatch.[8]
Duplicates should be discarded by the master. The slave must
respond to duplicates by repeating the last Database Description
packet that it had sent.
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When the router accepts a received Database Description Packet as the
next in sequence the packet contents are processed as follows. For
each LSA listed, the LSA's LS type is checked for validity. If the
LS type is unknown (e.g., not one of the LS types 1-5 defined by this
specification), or if this is an AS-external-LSA (LS type = 5) and
the neighbor is associated with a stub area, generate the neighbor
event SeqNumberMismatch and stop processing the packet. Otherwise,
the router looks up the LSA in its database to see whether it also
has an instance of the LSA. If it does not, or if the database copy
is less recent (see Section 13.1), the LSA is put on the Link state
request list so that it can be requested (immediately or at some
later time) in Link State Request Packets.
When the router accepts a received Database Description Packet as the
next in sequence, it also performs the following actions, depending
on whether it is master or slave:
Master
Increments the DD sequence number in the neighbor data structure.
If the router has already sent its entire sequence of Database
Description Packets, and the just accepted packet has the more bit
(M) set to 0, the neighbor event ExchangeDone is generated.
Otherwise, it should send a new Database Description to the slave.
Slave
Sets the DD sequence number in the neighbor data structure to the
DD sequence number appearing in the received packet. The slave
must send a Database Description Packet in reply. If the received
packet has the more bit (M) set to 0, and the packet to be sent by
the slave will also have the M-bit set to 0, the neighbor event
ExchangeDone is generated. Note that the slave always generates
this event before the master.
10.7. Receiving Link State Request Packets
This section explains the detailed processing of received Link State
Request packets. Received Link State Request Packets specify a list
of LSAs that the neighbor wishes to receive. Link State Request
Packets should be accepted when the neighbor is in states Exchange,
Loading, or Full. In all other states Link State Request Packets
should be ignored.
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Each LSA specified in the Link State Request packet should be located
in the router's database, and copied into Link State Update packets
for transmission to the neighbor. These LSAs should NOT be placed on
the Link state retransmission list for the neighbor. If an LSA
cannot be found in the database, something has gone wrong with the
Database Exchange process, and neighbor event BadLSReq should be
generated.
10.8. Sending Database Description Packets
This section describes how Database Description Packets are sent to a
neighbor. The Database Description packet's Interface MTU field is
set to the size of the largest IP datagram that can be sent out the
sending interface, without fragmentation. Common MTUs in use in the
Internet can be found in Table 7-1 of [Ref22]. Interface MTU should
be set to 0 in Database Description packets sent over virtual links.
The router's optional OSPF capabilities (see Section 4.5) are
transmitted to the neighbor in the Options field of the Database
Description packet. The router should maintain the same set of
optional capabilities throughout the Database Exchange and flooding
procedures. If for some reason the router's optional capabilities
change, the Database Exchange procedure should be restarted by
reverting to neighbor state ExStart. One optional capability is
defined in this specification (see Sections 4.5 and A.2). The E-bit
should be set if and only if the attached network belongs to a non-
stub area. Unrecognized bits in the Options field should be set to
zero. The sending of Database Description packets depends on the
neighbor's state. In state ExStart the router sends empty Database
Description packets, with the initialize (I), more (M) and master
(MS) bits set. These packets are retransmitted every RxmtInterval
seconds.
In state Exchange the Database Description Packets actually contain
summaries of the link state information contained in the router's
database. Each LSA in the area's link-state database (at the time
the neighbor transitions into Exchange state) is listed in the
neighbor Database summary list. Each new Database Description Packet
copies its DD sequence number from the neighbor data structure and
then describes the current top of the Database summary list. Items
are removed from the Database summary list when the previous packet
is acknowledged.
In state Exchange, the determination of when to send a Database
Description packet depends on whether the router is master or slave:
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Master
Database Description packets are sent when either a) the slave
acknowledges the previous Database Description packet by echoing
the DD sequence number or b) RxmtInterval seconds elapse without
an acknowledgment, in which case the previous Database Description
packet is retransmitted.
Slave
Database Description packets are sent only in response to Database
Description packets received from the master. If the Database
Description packet received from the master is new, a new Database
Description packet is sent, otherwise the previous Database
Description packet is resent.
In states Loading and Full the slave must resend its last Database
Description packet in response to duplicate Database Description
packets received from the master. For this reason the slave must
wait RouterDeadInterval seconds before freeing the last Database
Description packet. Reception of a Database Description packet from
the master after this interval will generate a SeqNumberMismatch
neighbor event.
10.9. Sending Link State Request Packets
In neighbor states Exchange or Loading, the Link state request list
contains a list of those LSAs that need to be obtained from the
neighbor. To request these LSAs, a router sends the neighbor the
beginning of the Link state request list, packaged in a Link State
Request packet.
When the neighbor responds to these requests with the proper Link
State Update packet(s), the Link state request list is truncated and
a new Link State Request packet is sent. This process continues
until the Link state request list becomes empty. Unsatisfied Link
State Request packets are retransmitted at intervals of RxmtInterval.
There should be at most one Link State Request packet outstanding at
any one time.
When the Link state request list becomes empty, and the neighbor
state is Loading (i.e., a complete sequence of Database Description
packets has been sent to and received from the neighbor), the Loading
Done neighbor event is generated.
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RFC 2178 OSPF Version 2 July 199710.10. An Example
Figure 14 shows an example of an adjacency forming. Routers RT1 and
RT2 are both connected to a broadcast network. It is assumed that
RT2 is the Designated Router for the network, and that RT2 has a
higher Router ID than Router RT1.
The neighbor state changes realized by each router are listed on the
sides of the figure.
At the beginning of Figure 14, Router RT1's interface to the network
becomes operational. It begins sending Hello Packets, although it
doesn't know the identity of the Designated Router or of any other
neighboring routers. Router RT2 hears this hello (moving the
neighbor to Init state), and in its next Hello Packet indicates that
it is itself the Designated Router and that it has heard Hello
Packets from RT1. This in turn causes RT1 to go to state ExStart, as
it starts to bring up the adjacency.
RT1 begins by asserting itself as the master. When it sees that RT2
is indeed the master (because of RT2's higher Router ID), RT1
transitions to slave state and adopts its neighbor's DD sequence
number. Database Description packets are then exchanged, with polls
coming from the master (RT2) and responses from the slave (RT1).
This sequence of Database Description Packets ends when both the poll
and associated response has the M-bit off.
In this example, it is assumed that RT2 has a completely up to date
database. In that case, RT2 goes immediately into Full state. RT1
will go into Full state after updating the necessary parts of its
database. This is done by sending Link State Request Packets, and
receiving Link State Update Packets in response. Note that, while
RT1 has waited until a complete set of Database Description Packets
has been received (from RT2) before sending any Link State Request
Packets, this need not be the case. RT1 could have interleaved the
sending of Link State Request Packets with the reception of Database
Description Packets.
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RFC 2178 OSPF Version 2 July 199711. The Routing Table Structure
The routing table data structure contains all the information
necessary to forward an IP data packet toward its destination. Each
routing table entry describes the collection of best paths to a
particular destination. When forwarding an IP data packet, the
routing table entry providing the best match for the packet's IP
destination is located. The matching routing table entry then
provides the next hop towards the packet's destination. OSPF also
provides for the existence of a default route (Destination ID =
DefaultDestination, Address Mask = 0x00000000). When the default
route exists, it matches all IP destinations (although any other
matching entry is a better match). Finding the routing table entry
that best matches an IP destination is further described in Section11.1.
There is a single routing table in each router. Two sample routing
tables are described in Sections 11.2 and 11.3. The building of the
routing table is discussed in Section 16.
The rest of this section defines the fields found in a routing table
entry. The first set of fields describes the routing table entry's
destination.
Destination Type
Destination type is either "network" or "router". Only network entries
are actually used when forwarding IP data traffic. Router routing
table entries are used solely as intermediate steps in the routing
table build process.
A network is a range of IP addresses, to which IP data traffic may be
forwarded. This includes IP networks (class A, B, or C), IP subnets,
IP supernets and single IP hosts. The default route also falls into
this category.
Router entries are kept for area border routers and AS boundary
routers. Routing table entries for area border routers are used when
calculating the inter-area routes (see Section 16.2), and when
maintaining configured virtual links (see Section 15). Routing table
entries for AS boundary routers are used when calculating the AS
external routes (see Section 16.4).
Destination ID
The destination's identifier or name. This depends on the
Destination Type. For networks, the identifier is their associated IP
address. For routers, the identifier is the OSPF Router ID.[9]
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Address Mask
Only defined for networks. The network's IP address together with its
address mask defines a range of IP addresses. For IP subnets, the
address mask is referred to as the subnet mask. For host routes, the
mask is "all ones" (0xffffffff).
Optional Capabilities
When the destination is a router this field indicates the optional
OSPF capabilities supported by the destination router. The only
optional capability defined by this specification is the ability to
process AS-external-LSAs. For a further discussion of OSPF's optional
capabilities, see Section 4.5.
The set of paths to use for a destination may vary based on the OSPF
area to which the paths belong. This means that there may be
multiple routing table entries for the same destination, depending on
the values of the next field.
Area
This field indicates the area whose link state information has led
to the routing table entry's collection of paths. This is called
the entry's associated area. For sets of AS external paths, this
field is not defined. For destinations of type "router", there
may be separate sets of paths (and therefore separate routing
table entries) associated with each of several areas. For example,
this will happen when two area border routers share multiple areas
in common. For destinations of type "network", only the set of
paths associated with the best area (the one providing the
preferred route) is kept.
The rest of the routing table entry describes the set of paths to the
destination. The following fields pertain to the set of paths as a
whole. In other words, each one of the paths contained in a routing
table entry is of the same path-type and cost (see below).
Path-type
There are four possible types of paths used to route traffic to
the destination, listed here in order of preference: intra-area,
inter-area, type 1 external or type 2 external. Intra-area paths
indicate destinations belonging to one of the router's attached
areas. Inter-area paths are paths to destinations in other OSPF
areas. These are discovered through the examination of received
summary-LSAs. AS external paths are paths to destinations
external to the AS. These are detected through the examination of
received AS-external-LSAs.
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Cost
The link state cost of the path to the destination. For all paths
except type 2 external paths this describes the entire path's
cost. For Type 2 external paths, this field describes the cost of
the portion of the path internal to the AS. This cost is
calculated as the sum of the costs of the path's constituent
links.
Type 2 cost
Only valid for type 2 external paths. For these paths, this field
indicates the cost of the path's external portion. This cost has
been advertised by an AS boundary router, and is the most
significant part of the total path cost. For example, a type 2
external path with type 2 cost of 5 is always preferred over a
path with type 2 cost of 10, regardless of the cost of the two
paths' internal components.
Link State Origin
Valid only for intra-area paths, this field indicates the LSA
(router-LSA or network-LSA) that directly references the
destination. For example, if the destination is a transit
network, this is the transit network's network-LSA. If the
destination is a stub network, this is the router-LSA for the
attached router. The LSA is discovered during the shortest-path
tree calculation (see Section 16.1). Multiple LSAs may reference
the destination, however a tie-breaking scheme always reduces the
choice to a single LSA. The Link State Origin field is not used by
the OSPF protocol, but it is used by the routing table calculation
in OSPF's Multicast routing extensions (MOSPF).
When multiple paths of equal path-type and cost exist to a
destination (called elsewhere "equal-cost" paths), they are stored in
a single routing table entry. Each one of the "equal-cost" paths is
distinguished by the following fields:
Next hop
The outgoing router interface to use when forwarding traffic to
the destination. On broadcast, Point-to-MultiPoint and NBMA
networks, the next hop also includes the IP address of the next
router (if any) in the path towards the destination.
Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary-LSA
or AS-external-LSA that led to this path.
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RFC 2178 OSPF Version 2 July 199711.1. Routing table lookup
When an IP data packet is received, an OSPF router finds the routing
table entry that best matches the packet's destination. This routing
table entry then provides the outgoing interface and next hop router
to use in forwarding the packet. This section describes the process
of finding the best matching routing table entry. The process
consists of a number of steps, wherein the collection of routing
table entries is progressively pruned. In the end, the single
routing table entry remaining is called the "best match".
Before the lookup begins, "discard" routing table entries should be
inserted into the routing table for each of the router's active area
address ranges (see Section 3.5). (An area range is considered
"active" if the range contains one or more networks reachable by
intra-area paths.) The destination of a "discard" entry is the set of
addresses described by its associated active area address range, and
the path type of each "discard" entry is set to "inter-area".[10]
Note that the steps described below may fail to produce a best match
routing table entry (i.e., all existing routing table entries are
pruned for some reason or another), or the best match routing table
entry may be one of the above "discard" routing table entries. In
these cases, the packet's IP destination is considered unreachable.
Instead of being forwarded, the packet should be discarded and an
ICMP destination unreachable message should be returned to the
packet's source.
(1) Select the complete set of "matching" routing table entries
from the routing table. Each routing table entry describes
a (set of) path(s) to a range of IP addresses. If the data
packet's IP destination falls into an entry's range of IP
addresses, the routing table entry is called a match. (It is
quite likely that multiple entries will match the data
packet. For example, a default route will match all
packets.)
(2) Reduce the set of matching entries to those having the most
preferential path-type (see Section 11). OSPF has a four
level hierarchy of paths. Intra-area paths are the most
preferred, followed in order by inter-area, type 1 external
and type 2 external paths.
(3) Select the remaining routing table entry that provides the
most specific (longest) match. Another way of saying this is
to choose the remaining entry that specifies the narrowest
range of IP addresses.[11] For example, the entry for the
address/mask pair of (128.185.1.0, 0xffffff00) is more
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specific than an entry for the pair (128.185.0.0,
0xffff0000). The default route is the least specific match,
since it matches all destinations.
11.2. Sample routing table, without areas
Consider the Autonomous System pictured in Figure 2. No OSPF areas
have been configured. A single metric is shown per outbound
interface. The calculation of Router RT6's routing table proceeds as
described in Section 2.2. The resulting routing table is shown in
Table 12. Destination types are abbreviated: Network as "N", Router
as "R".
There are no instances of multiple equal-cost shortest paths in this
example. Also, since there are no areas, there are no inter-area
paths.
Routers RT5 and RT7 are AS boundary routers. Intra-area routes have
been calculated to Routers RT5 and RT7. This allows external routes
to be calculated to the destinations advertised by RT5 and RT7 (i.e.,
Networks N12, N13, N14 and N15). It is assumed all AS-external-LSAs
originated by RT5 and RT7 are advertising type 1 external metrics.
This results in type 1 external paths being calculated to
destinations N12-N15.
11.3. Sample routing table, with areas
Consider the previous example, this time split into OSPF areas. An
OSPF area configuration is pictured in Figure 6. Router RT4's
routing table will be described for this area configuration. Router
RT4 has a connection to Area 1 and a backbone connection. This
causes Router RT4 to view the AS as the concatenation of the two
graphs shown in Figures 7 and 8. The resulting routing table is
displayed in Table 13.
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link are shown in Table 14.
12. Link State Advertisements (LSAs)
Each router in the Autonomous System originates one or more link
state advertisements (LSAs). This memo defines five distinct types
of LSAs, which are described in Section 4.3. The collection of LSAs
forms the link-state database. Each separate type of LSA has a
separate function. Router-LSAs and network-LSAs describe how an
area's routers and networks are interconnected. Summary-LSAs provide
a way of condensing an area's routing information. AS-external-LSAs
provide a way of transparently advertising externally-derived routing
information throughout the Autonomous System.
Each LSA begins with a standard 20-byte header. This LSA header is
discussed below.
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
R RT3 0 intra-area 1 * *
R RT10 0 intra-area 16 RT3 *
R RT11 0 intra-area 19 RT3 *
________________________________________________________________
N N9-N11,H1 0 inter-area 30 RT3 RT11
Table 14: Changes resulting from an
additional virtual link.
12.1. The LSA Header
The LSA header contains the LS type, Link State ID and Advertising
Router fields. The combination of these three fields uniquely
identifies the LSA.
There may be several instances of an LSA present in the Autonomous
System, all at the same time. It must then be determined which
instance is more recent. This determination is made by examining the
LS sequence, LS checksum and LS age fields. These fields are also
contained in the 20-byte LSA header.
Several of the OSPF packet types list LSAs. When the instance is not
important, an LSA is referred to by its LS type, Link State ID and
Advertising Router (see Link State Request Packets). Otherwise, the
LS sequence number, LS age and LS checksum fields must also be
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referenced.
A detailed explanation of the fields contained in the LSA header
follows.
12.1.1. LS age
This field is the age of the LSA in seconds. It should be processed
as an unsigned 16-bit integer. It is set to 0 when the LSA is
originated. It must be incremented by InfTransDelay on every hop of
the flooding procedure. LSAs are also aged as they are held in each
router's database.
The age of an LSA is never incremented past MaxAge. LSAs having age
MaxAge are not used in the routing table calculation. When an LSA's
age first reaches MaxAge, it is reflooded. An LSA of age MaxAge is
finally flushed from the database when it is no longer needed to
ensure database synchronization. For more information on the aging
of LSAs, consult Section 14.
The LS age field is examined when a router receives two instances of
an LSA, both having identical LS sequence numbers and LS checksums.
An instance of age MaxAge is then always accepted as most recent;
this allows old LSAs to be flushed quickly from the routing domain.
Otherwise, if the ages differ by more than MaxAgeDiff, the instance
having the smaller age is accepted as most recent.[12] See Section13.1 for more details.
12.1.2. Options
The Options field in the LSA header indicates which optional
capabilities are associated with the LSA. OSPF's optional
capabilities are described in Section 4.5. One optional capability is
defined by this specification, represented by the E-bit found in the
Options field. The unrecognized bits in the Options field should be
set to zero. The E-bit represents OSPF's ExternalRoutingCapability.
This bit should be set in all LSAs associated with the backbone, and
all LSAs associated with non-stub areas (see Section 3.6). It should
also be set in all AS-external-LSAs. It should be reset in all
router-LSAs, network-LSAs and summary-LSAs associated with a stub
area. For all LSAs, the setting of the E-bit is for informational
purposes only; it does not affect the routing table calculation.
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RFC 2178 OSPF Version 2 July 199712.1.3. LS type
The LS type field dictates the format and function of the LSA. LSAs
of different types have different names (e.g., router-LSAs or
network-LSAs). All LSA types defined by this memo, except the AS-
external-LSAs (LS type = 5), are flooded throughout a single area
only. AS-external-LSAs are flooded throughout the entire Autonomous
System, excepting stub areas (see Section 3.6). Each separate LSA
type is briefly described below in Table 15.
12.1.4. Link State ID
This field identifies the piece of the routing domain that is being
described by the LSA. Depending on the LSA's LS type, the Link State
ID takes on the values listed in Table 16.
Actually, for Type 3 summary-LSAs (LS type = 3) and AS-external-LSAs
(LS type = 5), the Link State ID may additionally have one or more of
the destination network's "host" bits set. For example, when
originating an AS-external-LSA for the network 10.0.0.0 with mask of
255.0.0.0, the Link State ID can be set to anything in the range
10.0.0.0 through 10.255.255.255 inclusive (although 10.0.0.0 should
be used whenever possible). The freedom to set certain host bits
allows a router to originate separate LSAs for two networks having
the same address but different masks. See Appendix E for details.
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LS Type LSA description
________________________________________________
1 These are the router-LSAs.
They describe the collected
states of the router's
interfaces. For more information,
consult Section 12.4.1.
________________________________________________
2 These are the network-LSAs.
They describe the set of routers
attached to the network. For
more information, consult
Section 12.4.2.
________________________________________________
3 or 4 These are the summary-LSAs.
They describe inter-area routes,
and enable the condensation of
routing information at area
borders. Originated by area border
routers, the Type 3 summary-LSAs
describe routes to networks while the
Type 4 summary-LSAs describe routes to
AS boundary routers.
________________________________________________
5 These are the AS-external-LSAs.
Originated by AS boundary routers,
they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS-external-LSA.
Table 15: OSPF link state advertisements (LSAs).
LS Type Link State ID
_______________________________________________
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.
Table 16: The LSA's Link State ID.
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When the LSA is describing a network (LS type = 2, 3 or 5), the
network's IP address is easily derived by masking the Link State ID
with the network/subnet mask contained in the body of the LSA. When
the LSA is describing a router (LS type = 1 or 4), the Link State ID
is always the described router's OSPF Router ID.
When an AS-external-LSA (LS Type = 5) is describing a default route,
its Link State ID is set to DefaultDestination (0.0.0.0).
12.1.5. Advertising Router
This field specifies the OSPF Router ID of the LSA's originator. For
router-LSAs, this field is identical to the Link State ID field.
Network-LSAs are originated by the network's Designated Router.
Summary-LSAs originated by area border routers. AS-external-LSAs are
originated by AS boundary routers.
12.1.6. LS sequence number
The sequence number field is a signed 32-bit integer. It is used to
detect old and duplicate LSAs. The space of sequence numbers is
linearly ordered. The larger the sequence number (when compared as
signed 32-bit integers) the more recent the LSA. To describe to
sequence number space more precisely, let N refer in the discussion
below to the constant 2**31.
The sequence number -N (0x80000000) is reserved (and unused). This
leaves -N + 1 (0x80000001) as the smallest (and therefore oldest)
sequence number; this sequence number is referred to as the constant
InitialSequenceNumber. A router uses InitialSequenceNumber the first
time it originates any LSA. Afterwards, the LSA's sequence number is
incremented each time the router originates a new instance of the
LSA. When an attempt is made to increment the sequence number past
the maximum value of N - 1 (0x7fffffff; also referred to as
MaxSequenceNumber), the current instance of the LSA must first be
flushed from the routing domain. This is done by prematurely aging
the LSA (see Section 14.1) and reflooding it. As soon as this flood
has been acknowledged by all adjacent neighbors, a new instance can
be originated with sequence number of InitialSequenceNumber.
The router may be forced to promote the sequence number of one of its
LSAs when a more recent instance of the LSA is unexpectedly received
during the flooding process. This should be a rare event. This may
indicate that an out-of-date LSA, originated by the router itself
before its last restart/reload, still exists in the Autonomous
System. For more information see Section 13.4.
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RFC 2178 OSPF Version 2 July 199712.1.7. LS checksum
This field is the checksum of the complete contents of the LSA,
excepting the LS age field. The LS age field is excepted so that an
LSA's age can be incremented without updating the checksum. The
checksum used is the same that is used for ISO connectionless
datagrams; it is commonly referred to as the Fletcher checksum. It
is documented in Annex B of [Ref6]. The LSA header also contains the
length of the LSA in bytes; subtracting the size of the LS age field
(two bytes) yields the amount of data to checksum.
The checksum is used to detect data corruption of an LSA. This
corruption can occur while an LSA is being flooded, or while it is
being held in a router's memory. The LS checksum field cannot take
on the value of zero; the occurrence of such a value should be
considered a checksum failure. In other words, calculation of the
checksum is not optional.
The checksum of an LSA is verified in two cases: a) when it is
received in a Link State Update Packet and b) at times during the
aging of the link state database. The detection of a checksum
failure leads to separate actions in each case. See Sections 13 and
14 for more details.
Whenever the LS sequence number field indicates that two instances of
an LSA are the same, the LS checksum field is examined. If there is
a difference, the instance with the larger LS checksum is considered
to be most recent.[13] See Section 13.1 for more details.
12.2. The link state database
A router has a separate link state database for every area to which
it belongs. All routers belonging to the same area have identical
link state databases for the area.
The databases for each individual area are always dealt with
separately. The shortest path calculation is performed separately
for each area (see Section 16). Components of the area link-state
database are flooded throughout the area only. Finally, when an
adjacency (belonging to Area A) is being brought up, only the
database for Area A is synchronized between the two routers.
The area database is composed of router-LSAs, network-LSAs and
summary-LSAs (all listed in the area data structure). In addition,
external routes (AS-external-LSAs) are included in all non-stub area
databases (see Section 3.6).
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An implementation of OSPF must be able to access individual pieces of
an area database. This lookup function is based on an LSA's LS type,
Link State ID and Advertising Router.[14] There will be a single
instance (the most up-to-date) of each LSA in the database. The
database lookup function is invoked during the LSA flooding procedure
(Section 13) and the routing table calculation (Section 16). In
addition, using this lookup function the router can determine whether
it has itself ever originated a particular LSA, and if so, with what
LS sequence number.
An LSA is added to a router's database when either a) it is received
during the flooding process (Section 13) or b) it is originated by
the router itself (Section 12.4). An LSA is deleted from a router's
database when either a) it has been overwritten by a newer instance
during the flooding process (Section 13) or b) the router originates
a newer instance of one of its self-originated LSAs (Section 12.4) or
c) the LSA ages out and is flushed from the routing domain (Section14).
Whenever an LSA is deleted from the database it must also be removed
from all neighbors' Link state retransmission lists (see Section 10).
12.3. Representation of TOS
For backward compatibility with previous versions of the OSPF
specification ([Ref9]), TOS-specific information can be included in
router-LSAs, summary-LSAs and AS-external-LSAs. The encoding of TOS
in OSPF LSAs is specified in Table 17. That table relates the OSPF
encoding to the IP packet header's TOS field (defined in [Ref12]).
The OSPF encoding is expressed as a decimal integer, and the IP
packet header's TOS field is expressed in the binary TOS values used
in [Ref12].
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OSPF encoding RFC 1349 TOS values
___________________________________________
0 0000 normal service
2 0001 minimize monetary cost
4 0010 maximize reliability
6 0011
8 0100 maximize throughput
10 0101
12 0110
14 0111
16 1000 minimize delay
18 1001
20 1010
22 1011
24 1100
26 1101
28 1110
30 1111
Table 17: Representing TOS in OSPF.
12.4. Originating LSAs
Into any given OSPF area, a router will originate several LSAs. Each
router originates a router-LSA. If the router is also the Designated
Router for any of the area's networks, it will originate network-LSAs
for those networks.
Area border routers originate a single summary-LSA for each known
inter-area destination. AS boundary routers originate a single AS-
external-LSA for each known AS external destination. Destinations
are advertised one at a time so that the change in any single route
can be flooded without reflooding the entire collection of routes.
During the flooding procedure, many LSAs can be carried by a single
Link State Update packet.
As an example, consider Router RT4 in Figure 6. It is an area border
router, having a connection to Area 1 and the backbone. Router RT4
originates 5 distinct LSAs into the backbone (one router-LSA, and one
summary-LSA for each of the networks N1-N4). Router RT4 will also
originate 8 distinct LSAs into Area 1 (one router-LSA and seven
summary-LSAs as pictured in Figure 7). If RT4 has been selected as
Designated Router for Network N3, it will also originate a network-
LSA for N3 into Area 1.
In this same figure, Router RT5 will be originating 3 distinct AS-
external-LSAs (one for each of the networks N12-N14). These will be
flooded throughout the entire AS, assuming that none of the areas
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have been configured as stubs. However, if area 3 has been
configured as a stub area, the AS-external-LSAs for networks N12-N14
will not be flooded into area 3 (see Section 3.6). Instead, Router
RT11 would originate a default summary- LSA that would be flooded
throughout area 3 (see Section 12.4.3). This instructs all of area
3's internal routers to send their AS external traffic to RT11.
Whenever a new instance of an LSA is originated, its LS sequence
number is incremented, its LS age is set to 0, its LS checksum is
calculated, and the LSA is added to the link state database and
flooded out the appropriate interfaces. See Section 13.2 for details
concerning the installation of the LSA into the link state database.
See Section 13.3 for details concerning the flooding of newly
originated LSAs.
The ten events that can cause a new instance of an LSA to be
originated are:
(1) The LS age field of one of the router's self-originated LSAs
reaches the value LSRefreshTime. In this case, a new
instance of the LSA is originated, even though the contents
of the LSA (apart from the LSA header) will be the same.
This guarantees periodic originations of all LSAs. This
periodic updating of LSAs adds robustness to the link state
algorithm. LSAs that solely describe unreachable
destinations should not be refreshed, but should instead be
flushed from the routing domain (see Section 14.1).
When whatever is being described by an LSA changes, a new LSA is
originated. However, two instances of the same LSA may not be
originated within the time period MinLSInterval. This may require
that the generation of the next instance be delayed by up to
MinLSInterval. The following events may cause the contents of an LSA
to change. These events should cause new originations if and only if
the contents of the new LSA would be different:
(2) An interface's state changes (see Section 9.1). This may
mean that it is necessary to produce a new instance of the
router-LSA.
(3) An attached network's Designated Router changes. A new
router-LSA should be originated. Also, if the router itself
is now the Designated Router, a new network-LSA should be
produced. If the router itself is no longer the Designated
Router, any network-LSA that it might have originated for
the network should be flushed from the routing domain (see
Section 14.1).
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(4) One of the neighboring routers changes to/from the FULL
state. This may mean that it is necessary to produce a new
instance of the router-LSA. Also, if the router is itself
the Designated Router for the attached network, a new
network-LSA should be produced.
The next four events concern area border routers only:
(5) An intra-area route has been added/deleted/modified in the
routing table. This may cause a new instance of a summary-
LSA (for this route) to be originated in each attached area
(possibly including the backbone).
(6) An inter-area route has been added/deleted/modified in the
routing table. This may cause a new instance of a summary-
LSA (for this route) to be originated in each attached area
(but NEVER for the backbone).
(7) The router becomes newly attached to an area. The router
must then originate summary-LSAs into the newly attached
area for all pertinent intra-area and inter-area routes in
the router's routing table. See Section 12.4.3 for more
details.
(8) When the state of one of the router's configured virtual
links changes, it may be necessary to originate a new
router-LSA into the virtual link's Transit area (see the
discussion of the router-LSA's bit V in Section 12.4.1), as
well as originating a new router-LSA into the backbone.
The last two events concern AS boundary routers (and former AS
boundary routers) only:
(9) An external route gained through direct experience with an
external routing protocol (like BGP) changes. This will
cause an AS boundary router to originate a new instance of
an AS-external-LSA.
(10)
A router ceases to be an AS boundary router, perhaps after
restarting. In this situation the router should flush all
AS-external-LSAs that it had previously originated. These
LSAs can be flushed via the premature aging procedure
specified in Section 14.1.
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The construction of each type of LSA is explained in detail below. In
general, these sections describe the contents of the LSA body (i.e.,
the part coming after the 20-byte LSA header). For information
concerning the building of the LSA header, see Section 12.1.
12.4.1. Router-LSAs
A router originates a router-LSA for each area that it belongs to.
Such an LSA describes the collected states of the router's links to
the area. The LSA is flooded throughout the particular area, and no
further. The format of a router-LSA is shown in Appendix A (Section
A.4.2). The first 20 bytes of the LSA consist of the generic LSA
header that was discussed in Section 12.1. router-LSAs have LS type
= 1.
A router also indicates whether it is an area border router, or an AS
boundary router, by setting the appropriate bits
....................................
. 192.1.2 Area 1 .
. + .
. | .
. | 3+---+1 .
. N1 |--|RT1|-----+ .
. | +---+ \ .
. | \ _______N3 .
. + \/ \ . 1+---+
. * 192.1.1 *------|RT4|
. + /\_______/ . +---+
. | / | .
. | 3+---+1 / | .
. N2 |--|RT2|-----+ 1| .
. | +---+ +---+8 . 6+---+
. | |RT3|----------------|RT6|
. + +---+ . +---+
. 192.1.3 |2 . 18.10.0.6|7
. | . |
. +------------+ .
. 192.1.4 (N4) .
....................................
Figure 15: Area 1 with IP addresses shown
(bit B and bit E, respectively) in its router-LSAs. This enables
paths to those types of routers to be saved in the routing table, for
later processing of summary-LSAs and AS-external-LSAs. Bit B should
be set whenever the router is actively attached to two or more areas,
even if the router is not currently attached to the OSPF backbone
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area. Bit E should never be set in a router-LSA for a stub area
(stub areas cannot contain AS boundary routers).
In addition, the router sets bit V in its router-LSA for Area A if
and only if the router is the endpoint of one or more fully adjacent
virtual links having Area A as their Transit area. The setting of bit
V enables other routers in Area A to discover whether the area
supports transit traffic (see TransitCapability in Section 6).
The router-LSA then describes the router's working connections (i.e.,
interfaces or links) to the area. Each link is typed according to
the kind of attached network. Each link is also labelled with its
Link ID. This Link ID gives a name to the entity that is on the
other end of the link. Table 18 summarizes the values used for the
Type and Link ID fields.
Link type Description Link ID
__________________________________________________
1 Point-to-point Neighbor Router ID
link
2 Link to transit Interface address of
network Designated Router
3 Link to stub IP network number
network
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the
router-LSA.
In addition, the Link Data field is specified for each link. This
field gives 32 bits of extra information for the link. For links to
transit networks, numbered point-to-point links and virtual links,
this field specifies the IP interface address of the associated
router interface (this is needed by the routing table calculation,
see Section 16.1.1). For links to stub networks, this field
specifies the stub network's IP address mask. For unnumbered point-
to-point links, the Link Data field should be set to the unnumbered
interface's MIB-II [Ref8] ifIndex value.
Finally, the cost of using the link for output is specified. The
output cost of a link is configurable. With the exception of links to
stub networks, the output cost must always be non-zero.
To further describe the process of building the list of link
descriptions, suppose a router wishes to build a router-LSA for Area
A. The router examines its collection of interface data structures.
For each interface, the following steps are taken:
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o If the attached network does not belong to Area A, no
links are added to the LSA, and the next interface should be
examined.
o If the state of the interface is Down, no links are added.
o If the state of the interface is Loopback, add a Type 3
link (stub network) as long as this is not an interface to an
unnumbered point-to-point network. The Link ID should be set to
the IP interface address, the Link Data set to the
mask 0xffffffff (indicating a host route), and the cost set to 0.
o Otherwise, the link descriptions added to the router-LSA
depend on the OSPF interface type. Link descriptions used for
point-to-point interfaces are specified in Section 12.4.1.1, for
virtual links in Section 12.4.1.2, for broadcast and NBMA
interfaces in 12.4.1.3, and for Point-to-MultiPoint interfaces in
12.4.1.4.
After consideration of all the router interfaces, host links are
added to the router-LSA by examining the list of attached hosts
belonging to Area A. A host route is represented as a Type 3 link
(stub network) whose Link ID is the host's IP address, Link Data is
the mask of all ones (0xffffffff), and cost the host's configured
cost (see Section C.7).
12.4.1.1. Describing point-to-point interfaces
For point-to-point interfaces, one or more link descriptions are
added to the router-LSA as follows:
o If the neighboring router is fully adjacent, add a
Type 1 link (point-to-point). The Link ID should be set to the
Router ID of the neighboring router. For numbered point-to-point
networks, the Link Data should specify the IP interface address.
For unnumbered point-to-point networks, the Link Data field
should specify the interface's MIB-II [Ref8] ifIndex value. The
cost should be set to the output cost of the point-to-point
interface.
o In addition, as long as the state of the interface
is "Point-to-Point" (and regardless of the neighboring router
state), a Type 3 link (stub network) should be added. There are
two forms that this stub link can take:
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Option 1
Assuming that the neighboring router's IP address is known, set
the Link ID of the Type 3 link to the neighbor's IP address, the
Link Data to the mask 0xffffffff (indicating a host route), and
the cost to the interface's configured output cost.[15]
Option 2
If a subnet has been assigned to the point-to-point link, set the
Link ID of the Type 3 link to the subnet's IP address, the Link
Data to the subnet's mask, and the cost to the interface's
configured output cost.[16]
12.4.1.2. Describing broadcast and NBMA interfaces
For operational broadcast and NBMA interfaces, a single link
description is added to the router-LSA as follows:
o If the state of the interface is Waiting, add a Type
3 link (stub network) with Link ID set to the IP network number
of the attached network, Link Data set to the attached network's
address mask, and cost equal to the interface's configured output
cost.
o Else, there has been a Designated Router elected for
the attached network. If the router is fully adjacent to the
Designated Router, or if the router itself is Designated Router
and is fully adjacent to at least one other router, add a single
Type 2 link (transit network) with Link ID set to the IP
interface address of the attached network's Designated Router
(which may be the router itself), Link Data set to the router's
own IP interface address, and cost equal to the interface's
configured output cost. Otherwise, add a link as if the
interface state were Waiting (see above).
12.4.1.3. Describing virtual links
For virtual links, a link description is added to the router-LSA only
when the virtual neighbor is fully adjacent. In this case, add a Type
4 link (virtual link) with Link ID set to the Router ID of the
virtual neighbor, Link Data set to the IP interface address
associated with the virtual link and cost set to the cost calculated
for the virtual link during the routing table calculation (see
Section 15).
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RFC 2178 OSPF Version 2 July 199712.4.1.4. Describing Point-to-MultiPoint interfaces
For operational Point-to-MultiPoint interfaces, one or more link
descriptions are added to the router-LSA as follows:
o A single Type 3 link (stub network) is added with
Link ID set to the router's own IP interface address, Link Data
set to the mask 0xffffffff (indicating a host route), and cost
set to 0.
o For each fully adjacent neighbor associated with the
interface, add an additional Type 1 link (point-to-point) with
Link ID set to the Router ID of the neighboring router, Link Data
set to the IP interface address and cost equal to the interface's
configured output cost.
12.4.1.5. Examples of router-LSAs
Consider the router-LSAs generated by Router RT3, as pictured in
Figure 6. The area containing Router RT3 (Area 1) has been redrawn,
with actual network addresses, in Figure 15. Assume that the last
byte of all of RT3's interface addresses is 3, giving it the
interface addresses 192.1.1.3 and 192.1.4.3, and that the other
routers have similar addressing schemes. In addition, assume that
all links are functional, and that Router IDs are assigned as the
smallest IP interface address.
RT3 originates two router-LSAs, one for Area 1 and one for the
backbone. Assume that Router RT4 has been selected as the Designated
router for network 192.1.1.0. RT3's router-LSA for Area 1 is then
shown below. It indicates that RT3 has two connections to Area 1,
the first a link to the transit network 192.1.1.0 and the second a
link to the stub network 192.1.4.0. Note that the transit network is
identified by the IP interface of its Designated Router (i.e., the
Link ID = 192.1.1.4 which is the Designated Router RT4's IP interface
to 192.1.1.0). Note also that RT3 has indicated that it is an area
border router.
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RFC 2178 OSPF Version 2 July 199712.4.2. Network-LSAs
A network-LSA is generated for every transit broadcast or NBMA
network. (A transit network is a network having two or more attached
routers). The network-LSA describes all the routers that are
attached to the network.
The Designated Router for the network originates the LSA. The
Designated Router originates the LSA only if it is fully adjacent to
at least one other router on the network. The network-LSA is flooded
throughout the area that contains the transit network, and no
further. The network-LSA lists those routers that are fully adjacent
to the Designated Router; each fully adjacent router is identified by
its OSPF Router ID. The Designated Router includes itself in this
list.
The Link State ID for a network-LSA is the IP interface address of
the Designated Router. This value, masked by the network's address
mask (which is also contained in the network-LSA) yields the
network's IP address.
A router that has formerly been the Designated Router for a network,
but is no longer, should flush the network-LSA that it had previously
originated. This LSA is no longer used in the routing table
calculation. It is flushed by prematurely incrementing the LSA's age
to MaxAge and reflooding (see Section 14.1). In addition, in those
rare cases where a router's Router ID has changed, any network-LSAs
that were originated with the router's previous Router ID must be
flushed. Since the router may have no idea what it's previous Router
ID might have been, these network-LSAs are indicated by having their
Link State ID equal to one of the router's IP interface addresses and
their Advertising Router equal to some value other than the router's
current Router ID (see Section 13.4 for more details).
12.4.2.1. Examples of network-LSAs
Again consider the area configuration in Figure 6. Network-LSAs are
originated for Network N3 in Area 1, Networks N6 and N8 in Area 2,
and Network N9 in Area 3. Assuming that Router RT4 has been selected
as the Designated Router for Network N3, the following network-LSA is
generated by RT4 on behalf of Network N3 (see Figure 15 for the
address assignments):
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RFC 2178 OSPF Version 2 July 1997
; Network-LSA for Network N3
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 2 ;indicates network-LSA
Link State ID = 192.1.1.4 ;IP address of Desig. Rtr.
Advertising Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID
Attached Router = 192.1.1.1 ;Router ID
Attached Router = 192.1.1.2 ;Router ID
Attached Router = 192.1.1.3 ;Router ID
12.4.3. Summary-LSAs
The destination described by a summary-LSA is either an IP network,
an AS boundary router or a range of IP addresses. Summary-LSAs are
flooded throughout a single area only. The destination described is
one that is external to the area, yet still belongs to the Autonomous
System.
Summary-LSAs are originated by area border routers. The precise
summary routes to advertise into an area are determined by examining
the routing table structure (see Section 11) in accordance with the
algorithm described below. Note that only intra-area routes are
advertised into the backbone, while both intra-area and inter-area
routes are advertised into the other areas.
To determine which routes to advertise into an attached Area A, each
routing table entry is processed as follows. Remember that each
routing table entry describes a set of equal-cost best paths to a
particular destination:
o Only Destination Types of network and AS boundary router
are advertised in summary-LSAs. If the routing table entry's
Destination Type is area border router, examine the next routing
table entry.
o AS external routes are never advertised in summary-LSAs.
If the routing table entry has Path-type of type 1 external or
type 2 external, examine the next routing table entry.
o Else, if the area associated with this set of paths is
the Area A itself, do not generate a summary-LSA for the
route.[17]
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RFC 2178 OSPF Version 2 July 1997
o Else, if the next hops associated with this set of paths
belong to Area A itself, do not generate a summary-LSA for the
route.[18] This is the logical equivalent of a Distance Vector
protocol's split horizon logic.
o Else, if the routing table cost equals or exceeds the
value LSInfinity, a summary-LSA cannot be generated for this
route.
o Else, if the destination of this route is an AS boundary
router, a summary-LSA should be originated if and only if the
routing table entry describes the preferred path to the AS
boundary router (see Step 3 of Section 16.4). If so, a Type 4
summary-LSA is originated for the destination, with Link State ID
equal to the AS boundary router's Router ID and metric equal to
the routing table entry's cost. Note: these LSAs should not be
generated if Area A has been configured as a stub area.
o Else, the Destination type is network. If this is an
inter-area route, generate a Type 3 summary-LSA for the
destination, with Link State ID equal to the network's address (if
necessary, the Link State ID can also have one or more of the
network's host bits set; see Appendix E for details) and metric
equal to the routing table cost.
o The one remaining case is an intra-area route to a network. This
means that the network is contained in one of the router's
directly attached areas. In general, this information must be
condensed before appearing in summary-LSAs. Remember that an area
has a configured list of address ranges, each range consisting of
an [address,mask] pair and a status indication of either Advertise
or DoNotAdvertise. At most a single Type 3 summary-LSA is
originated for each range. When the range's status indicates
Advertise, a Type 3 summary-LSA is generated with Link State ID
equal to the range's address (if necessary, the Link State ID can
also have one or more of the range's "host" bits set; see AppendixE for details) and cost equal to the largest cost of any of the
component networks. When the range's status indicates
DoNotAdvertise, the Type 3 summary-LSA is suppressed and the
component networks remain hidden from other areas.
By default, if a network is not contained in any explicitly
configured address range, a Type 3 summary-LSA is generated with Link
State ID equal to the network's address (if necessary, the Link State
ID can also have one or more of the network's "host" bits set; see
Appendix E for details) and metric equal to the network's routing
table cost.
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RFC 2178 OSPF Version 2 July 1997
If an area is capable of carrying transit traffic (i.e., its
TransitCapability is set to TRUE), routing information concerning
backbone networks should not be condensed before being summarized
into the area. Nor should the advertisement of backbone networks
into transit areas be suppressed. In other words, the backbone's
configured ranges should be ignored when originating summary-LSAs
into transit areas.
If a router advertises a summary-LSA for a destination which then
becomes unreachable, the router must then flush the LSA from the
routing domain by setting its age to MaxAge and reflooding (see
Section 14.1). Also, if the destination is still reachable, yet can
no longer be advertised according to the above procedure (e.g., it is
now an inter-area route, when it used to be an intra-area route
associated with some non-backbone area; it would thus no longer be
advertisable to the backbone), the LSA should also be flushed from
the routing domain.
12.4.3.1. Originating summary-LSAs into stub areas
The algorithm in Section 12.4.3 is optional when Area A is an OSPF
stub area. Area border routers connecting to a stub area can
originate summary-LSAs into the area according to the Section12.4.3's algorithm, or can choose to originate only a subset of the
summary-LSAs, possibly under configuration control. The fewer LSAs
originated, the smaller the stub area's link state database, further
reducing the demands on its routers' resources. However, omitting
LSAs may also lead to sub-optimal inter-area routing, although
routing will continue to function.
As specified in Section 12.4.3, Type 4 summary-LSAs (ASBR-summary-
LSAs) are never originated into stub areas.
In a stub area, instead of importing external routes each area border
router originates a "default summary-LSA" into the area. The Link
State ID for the default summary-LSA is set to DefaultDestination,
and the metric set to the (per-area) configurable parameter
StubDefaultCost. Note that StubDefaultCost need not be configured
identically in all of the stub area's area border routers.
12.4.3.2. Examples of summary-LSAs
Consider again the area configuration in Figure 6. Routers RT3, RT4,
RT7, RT10 and RT11 are all area border routers, and therefore are
originating summary-LSAs. Consider in particular Router RT4. Its
routing table was calculated as the example in Section 11.3. RT4
originates summary-LSAs into both the backbone and Area 1. Into the
backbone, Router RT4 originates separate LSAs for each of the
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RFC 2178 OSPF Version 2 July 1997
networks N1-N4. Into Area 1, Router RT4 originates separate LSAs for
networks N6-N8 and the AS boundary routers RT5,RT7. It also
condenses host routes Ia and Ib into a single summary-LSA. Finally,
the routes to networks N9,N10,N11 and Host H1 are advertised by a
single summary-LSA. This condensation was originally performed by
the router RT11.
These LSAs are illustrated graphically in Figures 7 and 8. Two of
the summary-LSAs originated by Router RT4 follow. The actual IP
addresses for the networks and routers in question have been assigned
in Figure 15.
; Summary-LSA for Network N1,
; originated by Router RT4 into the backbone
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 3 ;Type 3 summary-LSA
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
metric = 4
; Summary-LSA for AS boundary router RT7
; originated by Router RT4 into Area 1
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 4 ;Type 4 summary-LSA
Link State ID = Router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
metric = 14
12.4.4. AS-external-LSAs
AS-external-LSAs describe routes to destinations external to the
Autonomous System. Most AS-external-LSAs describe routes to specific
external destinations; in these cases the LSA's Link State ID is set
to the destination network's IP address (if necessary, the Link State
ID can also have one or more of the network's "host" bits set; see
Appendix E for details). However, a default route for the Autonomous
System can be described in an AS-external-LSA by setting the LSA's
Link State ID to DefaultDestination (0.0.0.0). AS-external-LSAs are
originated by AS boundary routers. An AS boundary router originates
a single AS-external-LSA for each external route that it has learned,
either through another routing protocol (such as BGP), or through
configuration information.
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AS-external-LSAs are the only type of LSAs that are flooded
throughout the entire Autonomous System; all other types of LSAs are
specific to a single area. However, AS-external-LSAs are not flooded
into/throughout stub areas (see Section 3.6). This enables a
reduction in link state database size for routers internal to stub
areas.
The metric that is advertised for an external route can be one of two
types. Type 1 metrics are comparable to the link state metric. Type
2 metrics are assumed to be larger than the cost of any intra-AS
path.
If a router advertises an AS-external-LSA for a destination which
then becomes unreachable, the router must then flush the LSA from the
routing domain by setting its age to MaxAge and reflooding (see
Section 14.1).
12.4.4.1. Examples of AS-external-LSAs
Consider once again the AS pictured in Figure 6. There are two AS
boundary routers: RT5 and RT7. Router RT5 originates three AS-
external-LSAs, for networks N12-N14. Router RT7 originates two AS-
external-LSAs, for networks N12 and N15. Assume that RT7 has learned
its route to N12 via BGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the following LSA for
N12:
; AS-external-LSA for Network N12,
; originated by Router RT7
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
metric = 2
Forwarding address = 0.0.0.0
In the above example, the forwarding address field has been set to
0.0.0.0, indicating that packets for the external destination should
be forwarded to the advertising OSPF router (RT7). This is not always
desirable. Consider the example pictured in Figure 16. There are
three OSPF routers (RTA, RTB and RTC) connected to a common network.
Only one of these routers, RTA, is exchanging BGP information with
the non-OSPF router RTX. RTA must then originate AS- external-LSAs
for those destinations it has learned from RTX. By using the AS-
external-LSA's forwarding address field, RTA can specify that packets
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RFC 2178 OSPF Version 2 July 1997
for these destinations be forwarded directly to RTX. Without this
feature, Routers RTB and RTC would take an extra hop to get to these
destinations.
Note that when the forwarding address field is non-zero, it should
point to a router belonging to another Autonomous System.
A forwarding address can also be specified for the default route. For
example, in figure 16 RTA may want to specify that all externally-
destined packets should by default be forwarded to its BGP peer RTX.
The resulting AS-external-LSA is pictured below. Note that the Link
State ID is set to DefaultDestination.
; Default route, originated by Router RTA
; Packets forwarded through RTX
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange BGP
information with RTX. In this case, RTA and RTB would originate the
same set of AS-external-LSAs. These LSAs, if they specify the same
metric, would be functionally equivalent since they would specify the
same destination and forwarding address (RTX). This leads to a clear
duplication of effort. If only one of RTA or RTB originated the set
of AS-external-LSAs, the routing would remain the same, and the size
of the link state database would decrease. However, it must be
unambiguously defined as to which router originates the LSAs
(otherwise neither may, or the identity of the originator may
oscillate). The following rule is thereby established: if two
routers, both reachable from one another, originate functionally
equivalent AS-external-LSAs (i.e., same destination, cost and non-
zero forwarding address), then the LSA originated by the router
having the highest OSPF Router ID is used. The router having the
lower OSPF Router ID can then flush its LSA. Flushing an LSA is
discussed in Section 14.1.
13. The Flooding Procedure
Link State Update packets provide the mechanism for flooding LSAs. A
Link State Update packet may contain several distinct LSAs, and
floods each LSA one hop further from its point of origination. To
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RFC 2178 OSPF Version 2 July 1997
make the flooding procedure reliable, each LSA must be acknowledged
separately. Acknowledgments are transmitted in Link State
Acknowledgment packets. Many separate acknowledgments can also be
grouped together into a single packet.
The flooding procedure starts when a Link State Update packet has
been received. Many consistency checks have been made on the
received packet before being handed to the flooding procedure (see
Section 8.2). In particular, the Link State Update packet has been
associated with a particular neighbor, and a particular area. If the
neighbor is in a lesser state than Exchange, the packet should be
dropped without further processing.
+
|
+---+.....|.BGP
|RTA|-----|.....+---+
+---+ |-----|RTX|
| +---+
+---+ |
|RTB|-----|
+---+ |
|
+---+ |
|RTC|-----|
+---+ |
|
+
Figure 16: Forwarding address example
All types of LSAs, other than AS-external-LSAs, are associated with a
specific area. However, LSAs do not contain an area field. An LSA's
area must be deduced from the Link State Update packet header.
For each LSA contained in a Link State Update packet, the following
steps are taken:
(1) Validate the LSA's LS checksum. If the checksum turns out to be
invalid, discard the LSA and get the next one from the Link
State Update packet.
(2) Examine the LSA's LS type. If the LS type is unknown, discard
the LSA and get the next one from the Link State Update Packet.
This specification defines LS types 1-5 (see Section 4.3).
(3) Else if this is an AS-external-LSA (LS type = 5), and the area
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RFC 2178 OSPF Version 2 July 1997
has been configured as a stub area, discard the LSA and get the
next one from the Link State Update Packet. AS-external-LSAs
are not flooded into/throughout stub areas (see Section 3.6).
(4) Else if the LSA's LS age is equal to MaxAge, and there is
currently no instance of the LSA in the router's link state
database, then take the following actions:
(a) Acknowledge the receipt of the LSA by sending a Link State
Acknowledgment packet back to the sending neighbor (see
Section 13.5).
(b) Purge all outstanding requests for equal or previous
instances of the LSA from the sending neighbor's Link State
Request list (see Section 10).
(c) If the sending neighbor is in state Exchange or in state
Loading, then install the MaxAge LSA in the link state
database. Otherwise, simply discard the LSA. In either
case, examine the next LSA (if any) listed in the Link State
Update packet.
(5) Otherwise, find the instance of this LSA that is currently
contained in the router's link state database. If there is no
database copy, or the received LSA is more recent than the
database copy (see Section 13.1 below for the determination of
which LSA is more recent) the following steps must be performed:
(a) If there is already a database copy, and if the database
copy was installed less than MinLSArrival seconds ago,
discard the new LSA (without acknowledging it) and examine
the next LSA (if any) listed in the Link State Update
packet.
(b) Otherwise immediately flood the new LSA out some subset of
the router's interfaces (see Section 13.3). In some cases
(e.g., the state of the receiving interface is DR and the
LSA was received from a router other than the Backup DR) the
LSA will be flooded back out the receiving interface. This
occurrence should be noted for later use by the
acknowledgment process (Section 13.5).
(c) Remove the current database copy from all neighbors' Link
state retransmission lists.
(d) Install the new LSA in the link state database (replacing
the current database copy). This may cause the routing
table calculation to be scheduled. In addition, timestamp
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RFC 2178 OSPF Version 2 July 1997
the new LSA with the current time (i.e., the time it was
received). The flooding procedure cannot overwrite the
newly installed LSA until MinLSArrival seconds have elapsed.
The LSA installation process is discussed further in Section13.2.
(e) Possibly acknowledge the receipt of the LSA by sending a
Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(f) If this new LSA indicates that it was originated by the
receiving router itself (i.e., is considered a self-
originated LSA), the router must take special action, either
updating the LSA or in some cases flushing it from the
routing domain. For a description of how self-originated
LSAs are detected and subsequently handled, see Section13.4.
(6) Else, if there is an instance of the LSA on the sending
neighbor's Link state request list, an error has occurred in the
Database Exchange process. In this case, restart the Database
Exchange process by generating the neighbor event BadLSReq for
the sending neighbor and stop processing the Link State Update
packet.
(7) Else, if the received LSA is the same instance as the database
copy (i.e., neither one is more recent) the following two steps
should be performed:
(a) If the LSA is listed in the Link state retransmission list
for the receiving adjacency, the router itself is expecting
an acknowledgment for this LSA. The router should treat the
received LSA as an acknowledgment by removing the LSA from
the Link state retransmission list. This is termed an
"implied acknowledgment". Its occurrence should be noted
for later use by the acknowledgment process (Section 13.5).
(b) Possibly acknowledge the receipt of the LSA by sending a
Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(8) Else, the database copy is more recent. If the database copy
has LS age equal to MaxAge and LS sequence number equal to
MaxSequenceNumber, simply discard the received LSA without
acknowledging it. (In this case, the LSA's LS sequence number is
wrapping, and the MaxSequenceNumber LSA must be completely
flushed before any new LSA instance can be introduced).
Otherwise, send the database copy back to the sending neighbor,
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RFC 2178 OSPF Version 2 July 1997
encapsulated within a Link State Update Packet. The Link State
Update Packet should be unicast to the neighbor. In so doing, do
not put the database copy of the LSA on the neighbor's link
state retransmission list, and do not acknowledge the received
(less recent) LSA instance.
13.1. Determining which LSA is newer
When a router encounters two instances of an LSA, it must determine
which is more recent. This occurred above when comparing a received
LSA to its database copy. This comparison must also be done during
the Database Exchange procedure which occurs during adjacency bring-
up.
An LSA is identified by its LS type, Link State ID and Advertising
Router. For two instances of the same LSA, the LS sequence number,
LS age, and LS checksum fields are used to determine which instance
is more recent:
o The LSA having the newer LS sequence number is more recent.
See Section 12.1.6 for an explanation of the LS sequence number
space. If both instances have the same LS sequence number, then:
o If the two instances have different LS checksums, then the
instance having the larger LS checksum (when considered as a 16-
bit unsigned integer) is considered more recent.
o Else, if only one of the instances has its LS age field set
to MaxAge, the instance of age MaxAge is considered to be more
recent.
o Else, if the LS age fields of the two instances differ by
more than MaxAgeDiff, the instance having the smaller (younger)
LS age is considered to be more recent.
o Else, the two instances are considered to be identical.
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RFC 2178 OSPF Version 2 July 199713.2. Installing LSAs in the database
Installing a new LSA in the database, either as the result of
flooding or a newly self-originated LSA, may cause the OSPF routing
table structure to be recalculated. The contents of the new LSA
should be compared to the old instance, if present. If there is no
difference, there is no need to recalculate the routing table. When
comparing an LSA to its previous instance, the following are all
considered to be differences in contents:
o The LSA's Options field has changed.
o One of the LSA instances has LS age set to MaxAge, and
the other does not.
o The length field in the LSA header has changed.
o The body of the LSA (i.e., anything outside the 20-byte
LSA header) has changed. Note that this excludes changes in LS
Sequence Number and LS Checksum.
If the contents are different, the following pieces of the routing
table must be recalculated, depending on the new LSA's LS type field:
Router-LSAs and network-LSAs
The entire routing table must be recalculated, starting with the
shortest path calculations for each area (not just the area whose
link-state database has changed). The reason that the shortest
path calculation cannot be restricted to the single changed area
has to do with the fact that AS boundary routers may belong to
multiple areas. A change in the area currently providing the best
route may force the router to use an intra-area route provided by
a different area.[19]
Summary-LSAs
The best route to the destination described by the summary-LSA
must be recalculated (see Section 16.5). If this destination is
an AS boundary router, it may also be necessary to re-examine all
the AS-external-LSAs.
AS-external-LSAs
The best route to the destination described by the AS-external-LSA
must be recalculated (see Section 16.6).
Also, any old instance of the LSA must be removed from the
database when the new LSA is installed. This old instance must
also be removed from all neighbors' Link state retransmission
lists (see Section 10).
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RFC 2178 OSPF Version 2 July 199713.3. Next step in the flooding procedure
When a new (and more recent) LSA has been received, it must be
flooded out some set of the router's interfaces. This section
describes the second part of flooding procedure (the first part being
the processing that occurred in Section 13), namely, selecting the
outgoing interfaces and adding the LSA to the appropriate neighbors'
Link state retransmission lists. Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link state
request lists.
This section is equally applicable to the flooding of an LSA that the
router itself has just originated (see Section 12.4).
For these LSAs, this section provides the entirety of the flooding
procedure (i.e., the processing of Section 13 is not performed,
since, for example, the LSA has not been received from a neighbor and
therefore does not need to be acknowledged).
Depending upon the LSA's LS type, the LSA can be flooded out only
certain interfaces. These interfaces, defined by the following, are
called the eligible interfaces:
AS-external-LSAs (LS Type = 5)
AS-external-LSAs are flooded throughout the entire AS, with the
exception of stub areas (see Section 3.6). The eligible
interfaces are all the router's interfaces, excluding virtual
links and those interfaces attaching to stub areas.
All other LS types
All other types are specific to a single area (Area A). The
eligible interfaces are all those interfaces attaching to the Area
A. If Area A is the backbone, this includes all the virtual
links.
Link state databases must remain synchronized over all adjacencies
associated with the above eligible interfaces. This is accomplished
by executing the following steps on each eligible interface. It
should be noted that this procedure may decide not to flood an LSA
out a particular interface, if there is a high probability that the
attached neighbors have already received the LSA. However, in these
cases the flooding procedure must be absolutely sure that the
neighbors eventually do receive the LSA, so the LSA is still added to
each adjacency's Link state retransmission list. For each eligible
interface:
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(1) Each of the neighbors attached to this interface are
examined, to determine whether they must receive the new
LSA. The following steps are executed for each neighbor:
(a) If the neighbor is in a lesser state than Exchange, it
does not participate in flooding, and the next neighbor
should be examined.
(b) Else, if the adjacency is not yet full (neighbor state
is Exchange or Loading), examine the Link state request
list associated with this adjacency. If there is an
instance of the new LSA on the list, it indicates that
the neighboring router has an instance of the LSA
already. Compare the new LSA to the neighbor's copy:
o If the new LSA is less recent, then examine the next
neighbor.
o If the two copies are the same instance, then delete
the LSA from the Link state request list, and
examine the next neighbor.[20]
o Else, the new LSA is more recent. Delete the LSA
from the Link state request list.
(c) If the new LSA was received from this neighbor, examine
the next neighbor.
(d) At this point we are not positive that the neighbor has
an up-to-date instance of this new LSA. Add the new LSA
to the Link state retransmission list for the adjacency.
This ensures that the flooding procedure is reliable;
the LSA will be retransmitted at intervals until an
acknowledgment is seen from the neighbor.
(2) The router must now decide whether to flood the new LSA out
this interface. If in the previous step, the LSA was NOT
added to any of the Link state retransmission lists, there
is no need to flood the LSA out the interface and the next
interface should be examined.
(3) If the new LSA was received on this interface, and it was
received from either the Designated Router or the Backup
Designated Router, chances are that all the neighbors have
received the LSA already. Therefore, examine the next
interface.
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(4) If the new LSA was received on this interface, and the
interface state is Backup (i.e., the router itself is the
Backup Designated Router), examine the next interface. The
Designated Router will do the flooding on this interface.
However, if the Designated Router fails the router (i.e.,
the Backup Designated Router) will end up retransmitting the
updates.
(5) If this step is reached, the LSA must be flooded out the
interface. Send a Link State Update packet (including the
new LSA as contents) out the interface. The LSA's LS age
must be incremented by InfTransDelay (which must be > 0)
when it is copied into the outgoing Link State Update packet
(until the LS age field reaches the maximum value of
MaxAge).
On broadcast networks, the Link State Update packets are
multicast. The destination IP address specified for the
Link State Update Packet depends on the state of the
interface. If the interface state is DR or Backup, the
address AllSPFRouters should be used. Otherwise, the
address AllDRouters should be used.
On non-broadcast networks, separate Link State Update
packets must be sent, as unicasts, to each adjacent neighbor
(i.e., those in state Exchange or greater). The destination
IP addresses for these packets are the neighbors' IP
addresses.
13.4. Receiving self-originated LSAs
It is a common occurrence for a router to receive self-originated
LSAs via the flooding procedure. A self-originated LSA is detected
when either 1) the LSA's Advertising Router is equal to the router's
own Router ID or 2) the LSA is a network-LSA and its Link State ID is
equal to one of the router's own IP interface addresses.
However, if the received self-originated LSA is newer than the last
instance that the router actually originated, the router must take
special action. The reception of such an LSA indicates that there
are LSAs in the routing domain that were originated by the router
before the last time it was restarted. In most cases, the router
must then advance the LSA's LS sequence number one past the received
LS sequence number, and originate a new instance of the LSA.
It may be the case the router no longer wishes to originate the
received LSA. Possible examples include: 1) the LSA is a summary-LSA
or AS-external-LSA and the router no longer has an (advertisable)
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route to the destination, 2) the LSA is a network-LSA but the router
is no longer Designated Router for the network or 3) the LSA is a
network-LSA whose Link State ID is one of the router's own IP
interface addresses but whose Advertising Router is not equal to the
router's own Router ID (this latter case should be rare, and it
indicates that the router's Router ID has changed since originating
the LSA). In all these cases, instead of updating the LSA, the LSA
should be flushed from the routing domain by incrementing the
received LSA's LS age to MaxAge and reflooding (see Section 14.1).
13.5. Sending Link State Acknowledgment packets
Each newly received LSA must be acknowledged. This is usually done
by sending Link State Acknowledgment packets. However,
acknowledgments can also be accomplished implicitly by sending Link
State Update packets (see step 7a of Section 13).
Many acknowledgments may be grouped together into a single Link State
Acknowledgment packet. Such a packet is sent back out the interface
which received the LSAs. The packet can be sent in one of two ways:
delayed and sent on an interval timer, or sent directly (as a
unicast) to a particular neighbor. The particular acknowledgment
strategy used depends on the circumstances surrounding the receipt of
the LSA.
Sending delayed acknowledgments accomplishes several things: 1) it
facilitates the packaging of multiple acknowledgments in a single
Link State Acknowledgment packet, 2) it enables a single Link State
Acknowledgment packet to indicate acknowledgments to several
neighbors at once (through multicasting) and 3) it randomizes the
Link State Acknowledgment packets sent by the various routers
attached to a common network. The fixed interval between a router's
delayed transmissions must be short (less than RxmtInterval) or
needless retransmissions will ensue.
Direct acknowledgments are sent to a particular neighbor in response
to the receipt of duplicate LSAs. These acknowledgments are sent as
unicasts, and are sent immediately when the duplicate is received.
The precise procedure for sending Link State Acknowledgment packets
is described in Table 19. The circumstances surrounding the receipt
of the LSA are listed in the left column. The acknowledgment action
then taken is listed in one of the two right columns. This action
depends on the state of the concerned interface; interfaces in state
Backup behave differently from interfaces in all other states.
Delayed acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment packets
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as multicasts. The Destination IP address used depends on the state
of the interface. If the interface state is DR or Backup, the
destination AllSPFRouters is used. In all other states, the
destination AllDRouters is used. On non-broadcast networks, delayed
Link State Acknowledgment packets must be unicast separately over
each adjacency (i.e., neighbor whose state is >= Exchange).
Action taken in state
Circumstances Backup All other states
_______________________________________________________________
LSA has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
_______________________________________________________________
LSA is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received
was not flooded from Designated
back out receiving Router, otherwise
interface do nothing
_______________________________________________________________
LSA is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent.
treated as an im- tisement received
plied acknowledg- from Designated
ment (see Section Router, otherwise
13, step 7a). do nothing
_______________________________________________________________
LSA is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
_______________________________________________________________
LSA's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent.
MaxAge, and there is
no current instance
of the LSA
in the link state
database (see
Section 13, step 4).
Table 19: Sending link state acknowledgments.
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The reasoning behind sending the above packets as multicasts is best
explained by an example. Consider the network configuration depicted
in Figure 15. Suppose RT4 has been elected as Designated Router, and
RT3 as Backup Designated Router for the network N3. When Router RT4
floods a new LSA to Network N3, it is received by routers RT1, RT2,
and RT3. These routers will not flood the LSA back onto net N3, but
they still must ensure that their link-state databases remain
synchronized with their adjacent neighbors. So RT1, RT2, and RT4 are
waiting to see an acknowledgment from RT3. Likewise, RT4 and RT3 are
both waiting to see acknowledgments from RT1 and RT2. This is best
achieved by sending the acknowledgments as multicasts.
The reason that the acknowledgment logic for Backup DRs is slightly
different is because they perform differently during the flooding of
LSAs (see Section 13.3, step 4).
13.6. Retransmitting LSAs
LSAs flooded out an adjacency are placed on the adjacency's Link
state retransmission list. In order to ensure that flooding is
reliable, these LSAs are retransmitted until they are acknowledged.
The length of time between retransmissions is a configurable per-
interface value, RxmtInterval. If this is set too low for an
interface, needless retransmissions will ensue. If the value is set
too high, the speed of the flooding, in the face of lost packets, may
be affected.
Several retransmitted LSAs may fit into a single Link State Update
packet. When LSAs are to be retransmitted, only the number fitting
in a single Link State Update packet should be sent. Another packet
of retransmissions can be sent whenever some of the LSAs are
acknowledged, or on the next firing of the retransmission timer.
Link State Update Packets carrying retransmissions are always sent as
unicasts (directly to the physical address of the neighbor). They
are never sent as multicasts. Each LSA's LS age must be incremented
by InfTransDelay (which must be > 0) when it is copied into the
outgoing Link State Update packet (until the LS age field reaches the
maximum value of MaxAge).
If an adjacent router goes down, retransmissions may occur until the
adjacency is destroyed by OSPF's Hello Protocol. When the adjacency
is destroyed, the Link state retransmission list is cleared.
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Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding procedure.
In particular, it has been associated with a particular neighbor. If
this neighbor is in a lesser state than Exchange, the Link State
Acknowledgment packet is discarded.
Otherwise, for each acknowledgment in the Link State Acknowledgment
packet, the following steps are performed:
o Does the LSA acknowledged have an instance on the Link state
retransmission list for the neighbor? If not, examine the
next acknowledgment. Otherwise:
o If the acknowledgment is for the same instance that is
contained on the list, remove the item from the list and
examine the next acknowledgment. Otherwise:
o Log the questionable acknowledgment, and examine the next
one.
14. Aging The Link State Database
Each LSA has an LS age field. The LS age is expressed in seconds.
An LSA's LS age field is incremented while it is contained in a
router's database. Also, when copied into a Link State Update Packet
for flooding out a particular interface, the LSA's LS age is
incremented by InfTransDelay.
An LSA's LS age is never incremented past the value MaxAge. LSAs
having age MaxAge are not used in the routing table calculation. As
a router ages its link state database, an LSA's LS age may reach
MaxAge.[21] At this time, the router must attempt to flush the LSA
from the routing domain. This is done simply by reflooding the
MaxAge LSA just as if it was a newly originated LSA (see Section13.3).
When creating a Database summary list for a newly forming adjacency,
any MaxAge LSAs present in the link state database are added to the
neighbor's Link state retransmission list instead of the neighbor's
Database summary list. See Section 10.3 for more details.
A MaxAge LSA must be removed immediately from the router's link state
database as soon as both a) it is no longer contained on any neighbor
Link state retransmission lists and b) none of the router's neighbors
are in states Exchange or Loading.
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When, in the process of aging the link state database, an LSA's LS
age hits a multiple of CheckAge, its LS checksum should be verified.
If the LS checksum is incorrect, a program or memory error has been
detected, and at the very least the router itself should be
restarted.
14.1. Premature aging of LSAs
An LSA can be flushed from the routing domain by setting its LS age
to MaxAge and reflooding the LSA. This procedure follows the same
course as flushing an LSA whose LS age has naturally reached the
value MaxAge (see Section 14). In particular, the MaxAge LSA is
removed from the router's link state database as soon as a) it is no
longer contained on any neighbor Link state retransmission lists and
b) none of the router's neighbors are in states Exchange or Loading.
We call the setting of an LSA's LS age to MaxAge "premature aging".
Premature aging is used when it is time for a self-originated LSA's
sequence number field to wrap. At this point, the current LSA
instance (having LS sequence number MaxSequenceNumber) must be
prematurely aged and flushed from the routing domain before a new
instance with sequence number equal to InitialSequenceNumber can be
originated. See Section 12.1.6 for more information.
Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its AS-
external-LSA from the routing domain via premature aging. This
procedure is preferable to the alternative, which is to originate a
new LSA for the destination specifying a metric of LSInfinity.
Premature aging is also be used when unexpectedly receiving self-
originated LSAs during the flooding procedure (see Section 13.4).
A router may only prematurely age its own self-originated LSAs. The
router may not prematurely age LSAs that have been originated by
other routers. An LSA is considered self- originated when either 1)
the LSA's Advertising Router is equal to the router's own Router ID
or 2) the LSA is a network-LSA and its Link State ID is equal to one
of the router's own IP interface addresses.
15. Virtual Links
The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
or some areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can be
configured through non-backbone areas. Virtual links serve to
connect physically separate components of the backbone. The two
endpoints of a virtual link are area border routers. The virtual
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link must be configured in both routers. The configuration
information in each router consists of the other virtual endpoint
(the other area border router), and the non-backbone area the two
routers have in common (called the Transit area). Virtual links
cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-
point network belonging to the backbone and joining the two area
border routers. An attempt is made to establish an adjacency over
the virtual link. When this adjacency is established, the virtual
link will be included in backbone router-LSAs, and OSPF packets
pertaining to the backbone area will flow over the adjacency. Such
an adjacency has been referred to in this document as a "virtual
adjacency".
In each endpoint router, the cost and viability of the virtual link
is discovered by examining the routing table entry for the other
endpoint router. (The entry's associated area must be the configured
Transit area). This is called the virtual link's corresponding
routing table entry. The InterfaceUp event occurs for a virtual link
when its corresponding routing table entry becomes reachable.
Conversely, the InterfaceDown event occurs when its routing table
entry becomes unreachable. In other words, the virtual link's
viability is determined by the existence of an intra-area path,
through the Transit area, between the two endpoints. Note that a
virtual link whose underlying path has cost greater than hexadecimal
0xffff (the maximum size of an interface cost in a router-LSA) should
be considered inoperational (i.e., treated the same as if the path
did not exist).
The other details concerning virtual links are as follows:
o AS-external-LSAs are NEVER flooded over virtual adjacencies. This
would be duplication of effort, since the same AS-external-LSAs are
already flooded throughout the virtual link's Transit area. For this
same reason, AS-external-LSAs are not summarized over virtual
adjacencies during the Database Exchange process.
o The cost of a virtual link is NOT configured. It is defined to be
the cost of the intra-area path between the two defining area border
routers. This cost appears in the virtual link's corresponding
routing table entry. When the cost of a virtual link changes, a new
router-LSA should be originated for the backbone area.
o Just as the virtual link's cost and viability are determined by the
routing table build process (through construction of the routing
table entry for the other endpoint), so are the IP interface address
for the virtual interface and the virtual neighbor's IP address.
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These are used when sending OSPF protocol packets over the virtual
link. Note that when one (or both) of the virtual link endpoints
connect to the Transit area via an unnumbered point-to-point link, it
may be impossible to calculate either the virtual interface's IP
address and/or the virtual neighbor's IP address, thereby causing the
virtual link to fail.
o In each endpoint's router-LSA for the backbone, the virtual link is
represented as a Type 4 link whose Link ID is set to the virtual
neighbor's OSPF Router ID and whose Link Data is set to the virtual
interface's IP address. See Section 12.4.1 for more information.
o A non-backbone area can carry transit data traffic (i.e., is
considered a "transit area") if and only if it serves as the Transit
area for one or more fully adjacent virtual links (see
TransitCapability in Sections 6 and 16.1). Such an area requires
special treatment when summarizing backbone networks into it (see
Section 12.4.3), and during the routing calculation (see Section16.3).
o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the expected
round-trip delay between the two routers. This may be hard to
estimate for a virtual link; it is better to err on the side of
making it too large.
16. Calculation of the routing table
This section details the OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step. At
each step, the router must access individual pieces of the link state
databases (e.g., a router-LSA originated by a certain router). This
access is performed by the lookup function discussed in Section 12.2.
The lookup process may return an LSA whose LS age is equal to MaxAge.
Such an LSA should not be used in the routing table calculation, and
is treated just as if the lookup process had failed.
The OSPF routing table's organization is explained in Section 11.
Two examples of the routing table build process are presented in
Sections 11.2 and 11.3. This process can be broken into the
following steps:
(1) The present routing table is invalidated. The routing table is
built again from scratch. The old routing table is saved so
that changes in routing table entries can be identified.
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(2) The intra-area routes are calculated by building the shortest-
path tree for each attached area. In particular, all routing
table entries whose Destination Type is "area border router" are
calculated in this step. This step is described in two parts.
At first the tree is constructed by only considering those links
between routers and transit networks. Then the stub networks
are incorporated into the tree. During the area's shortest-path
tree calculation, the area's TransitCapability is also
calculated for later use in Step 4.
(3) The inter-area routes are calculated, through examination of
summary-LSAs. If the router is attached to multiple areas
(i.e., it is an area border router), only backbone summary-LSAs
are examined.
(4) In area border routers connecting to one or more transit areas
(i.e, non-backbone areas whose TransitCapability is found to be
TRUE), the transit areas' summary-LSAs are examined to see
whether better paths exist using the transit areas than were
found in Steps 2-3 above.
(5) Routes to external destinations are calculated, through
examination of AS-external-LSAs. The locations of the AS
boundary routers (which originate the AS-external-LSAs) have
been determined in steps 2-4.
Steps 2-5 are explained in further detail below.
Changes made to routing table entries as a result of these
calculations can cause the OSPF protocol to take further actions.
For example, a change to an intra-area route will cause an area
border router to originate new summary-LSAs (see Section 12.4). See
Section 16.7 for a complete list of the OSPF protocol actions
resulting from routing table changes.
16.1. Calculating the shortest-path tree for an area
This calculation yields the set of intra-area routes associated with
an area (called hereafter Area A). A router calculates the
shortest-path tree using itself as the root.[22] The formation of the
shortest path tree is done here in two stages. In the first stage,
only links between routers and transit networks are considered.
Using the Dijkstra algorithm, a tree is formed from this subset of
the link state database. In the second stage, leaves are added to
the tree by considering the links to stub networks.
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The procedure will be explained using the graph terminology that was
introduced in Section 2. The area's link state database is
represented as a directed graph. The graph's vertices are routers,
transit networks and stub networks. The first stage of the procedure
concerns only the transit vertices (routers and transit networks) and
their connecting links. Throughout the shortest path calculation,
the following data is also associated with each transit vertex:
Vertex (node) ID
A 32-bit number uniquely identifying the vertex. For router
vertices this is the router's OSPF Router ID. For network
vertices, this is the IP address of the network's Designated
Router.
An LSA
Each transit vertex has an associated LSA. For router
vertices, this is a router-LSA. For transit networks, this
is a network-LSA (which is actually originated by the
network's Designated Router). In any case, the LSA's Link
State ID is always equal to the above Vertex ID.
List of next hops
The list of next hops for the current set of shortest paths
from the root to this vertex. There can be multiple
shortest paths due to the equal-cost multipath capability.
Each next hop indicates the outgoing router interface to use
when forwarding traffic to the destination. On broadcast,
Point-to-MultiPoint and NBMA networks, the next hop also
includes the IP address of the next router (if any) in the
path towards the destination.
Distance from root
The link state cost of the current set of shortest paths
from the root to the vertex. The link state cost of a path
is calculated as the sum of the costs of the path's
constituent links (as advertised in router-LSAs and
network-LSAs). One path is said to be "shorter" than
another if it has a smaller link state cost.
The first stage of the procedure (i.e., the Dijkstra algorithm) can
now be summarized as follows. At each iteration of the algorithm,
there is a list of candidate vertices. Paths from the root to these
vertices have been found, but not necessarily the shortest ones.
However, the paths to the candidate vertex that is closest to the
root are guaranteed to be shortest; this vertex is added to the
shortest-path tree, removed from the candidate list, and its adjacent
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vertices are examined for possible addition to/modification of the
candidate list. The algorithm then iterates again. It terminates
when the candidate list becomes empty.
The following steps describe the algorithm in detail. Remember that
we are computing the shortest path tree for Area A. All references
to link state database lookup below are from Area A's database.
(1) Initialize the algorithm's data structures. Clear the list
of candidate vertices. Initialize the shortest-path tree to
only the root (which is the router doing the calculation).
Set Area A's TransitCapability to FALSE.
(2) Call the vertex just added to the tree vertex V. Examine
the LSA associated with vertex V. This is a lookup in the
Area A's link state database based on the Vertex ID. If
this is a router-LSA, and bit V of the router-LSA (see
Section A.4.2) is set, set Area A's TransitCapability to
TRUE. In any case, each link described by the LSA gives the
cost to an adjacent vertex. For each described link, (say
it joins vertex V to vertex W):
(a) If this is a link to a stub network, examine the next
link in V's LSA. Links to stub networks will be
considered in the second stage of the shortest path
calculation.
(b) Otherwise, W is a transit vertex (router or transit
network). Look up the vertex W's LSA (router-LSA or
network-LSA) in Area A's link state database. If the
LSA does not exist, or its LS age is equal to MaxAge, or
it does not have a link back to vertex V, examine the
next link in V's LSA.[23]
(c) If vertex W is already on the shortest-path tree,
examine the next link in the LSA.
(d) Calculate the link state cost D of the resulting path
from the root to vertex W. D is equal to the sum of the
link state cost of the (already calculated) shortest
path to vertex V and the advertised cost of the link
between vertices V and W. If D is:
o Greater than the value that already appears for
vertex W on the candidate list, then examine the
next link.
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o Equal to the value that appears for vertex W on the
candidate list, calculate the set of next hops that
result from using the advertised link. Input to
this calculation is the destination (W), and its
parent (V). This calculation is shown in Section16.1.1. This set of hops should be added to the
next hop values that appear for W on the candidate
list.
o Less than the value that appears for vertex W on the
candidate list, or if W does not yet appear on the
candidate list, then set the entry for W on the
candidate list to indicate a distance of D from the
root. Also calculate the list of next hops that
result from using the advertised link, setting the
next hop values for W accordingly. The next hop
calculation is described in Section 16.1.1; it takes
as input the destination (W) and its parent (V).
(3) If at this step the candidate list is empty, the shortest-
path tree (of transit vertices) has been completely built
and this stage of the procedure terminates. Otherwise,
choose the vertex belonging to the candidate list that is
closest to the root, and add it to the shortest-path tree
(removing it from the candidate list in the process). Note
that when there is a choice of vertices closest to the root,
network vertices must be chosen before router vertices in
order to necessarily find all equal-cost paths. This is
consistent with the tie-breakers that were introduced in the
modified Dijkstra algorithm used by OSPF's Multicast routing
extensions (MOSPF).
(4) Possibly modify the routing table. For those routing table
entries modified, the associated area will be set to Area A,
the path type will be set to intra-area, and the cost will
be set to the newly discovered shortest path's calculated
distance.
If the newly added vertex is an area border router or AS
boundary router, a routing table entry is added whose
destination type is "router". The Options field found in
the associated router-LSA is copied into the routing table
entry's Optional capabilities field. Call the newly added
vertex Router X. If Router X is the endpoint of one of the
calculating router's virtual links, and the virtual link
uses Area A as Transit area: the virtual link is declared
up, the IP address of the virtual interface is set to the IP
address of the outgoing interface calculated above for
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Router X, and the virtual neighbor's IP address is set to
Router X's interface address (contained in Router X's
router-LSA) that points back to the root of the shortest-
path tree; equivalently, this is the interface that points
back to Router X's parent vertex on the shortest-path tree
(similar to the calculation in Section 16.1.1).
If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
Destination ID is the IP network number, which can be
obtained by masking the Vertex ID (Link State ID) with its
associated subnet mask (found in the body of the associated
network-LSA). If the routing table entry already exists
(i.e., there is already an intra-area route to the
destination installed in the routing table), multiple
vertices have mapped to the same IP network. For example,
this can occur when a new Designated Router is being
established. In this case, the current routing table entry
should be overwritten if and only if the newly found path is
just as short and the current routing table entry's Link
State Origin has a smaller Link State ID than the newly
added vertex' LSA.
If there is no routing table entry for the network (the
usual case), a routing table entry for the IP network should
be added. The routing table entry's Link State Origin
should be set to the newly added vertex' LSA.
(5) Iterate the algorithm by returning to Step 2.
The stub networks are added to the tree in the procedure's second
stage. In this stage, all router vertices are again examined. Those
that have been determined to be unreachable in the above first phase
are discarded. For each reachable router vertex (call it V), the
associated router-LSA is found in the link state database. Each stub
network link appearing in the LSA is then examined, and the following
steps are executed:
(1) Calculate the distance D of stub network from the root. D
is equal to the distance from the root to the router vertex
(calculated in stage 1), plus the stub network link's
advertised cost. Compare this distance to the current best
cost to the stub network. This is done by looking up the
stub network's current routing table entry. If the
calculated distance D is larger, go on to examine the next
stub network link in the LSA.
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(2) If this step is reached, the stub network's routing table
entry must be updated. Calculate the set of next hops that
would result from using the stub network link. This
calculation is shown in Section 16.1.1; input to this
calculation is the destination (the stub network) and the
parent vertex (the router vertex). If the distance D is the
same as the current routing table cost, simply add this set
of next hops to the routing table entry's list of next hops.
In this case, the routing table already has a Link State
Origin. If this Link State Origin is a router-LSA whose
Link State ID is smaller than V's Router ID, reset the Link
State Origin to V's router-LSA.
Otherwise D is smaller than the routing table cost.
Overwrite the current routing table entry by setting the
routing table entry's cost to D, and by setting the entry's
list of next hops to the newly calculated set. Set the
routing table entry's Link State Origin to V's router-LSA.
Then go on to examine the next stub network link.
For all routing table entries added/modified in the second stage, the
associated area will be set to Area A and the path type will be set to
intra-area. When the list of reachable router-LSAs is exhausted, the
second stage is completed. At this time, all intra-area routes
associated with Area A have been determined.
The specification does not require that the above two stage method be
used to calculate the shortest path tree. However, if another
algorithm is used, an identical tree must be produced. For this
reason, it is important to note that links between transit vertices
must be bidirectional in order to be included in the above tree. It
should also be mentioned that more efficient algorithms exist for
calculating the tree; for example, the incremental SPF algorithm
described in [Ref1].
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RFC 2178 OSPF Version 2 July 199716.1.1. The next hop calculation
This section explains how to calculate the current set of next hops
to use for a destination. Each next hop consists of the outgoing
interface to use in forwarding packets to the destination together
with the IP address of the next hop router (if any). The next hop
calculation is invoked each time a shorter path to the destination is
discovered. This can happen in either stage of the shortest-path
tree calculation (see Section 16.1). In stage 1 of the shortest-path
tree calculation a shorter path is found as the destination is added
to the candidate list, or when the destination's entry on the
candidate list is modified (Step 2d of Stage 1). In stage 2 a
shorter path is discovered each time the destination's routing table
entry is modified (Step 2 of Stage 2).
The set of next hops to use for the destination may be recalculated
several times during the shortest-path tree calculation, as shorter
and shorter paths are discovered. In the end, the destination's
routing table entry will always reflect the next hops resulting from
the absolute shortest path(s).
Input to the next hop calculation is a) the destination and b) its
parent in the current shortest path between the root (the calculating
router) and the destination. The parent is always a transit vertex
(i.e., always a router or a transit network).
If there is at least one intervening router in the current shortest
path between the destination and the root, the destination simply
inherits the set of next hops from the parent. Otherwise, there are
two cases. In the first case, the parent vertex is the root (the
calculating router itself). This means that the destination is
either a directly connected network or directly connected router.
The outgoing interface in this case is simply the OSPF interface
connecting to the destination network/router. If the destination is a
router which connects to the calculating router via a Point-to-
MultiPoint network, the destination's next hop IP address(es) can be
determined by examining the destination's router-LSA: each link
pointing back to the calculating router and having a Link Data field
belonging to the Point-to-MultiPoint network provides an IP address
of the next hop router. If the destination is a directly connected
network, or a router which connects to the calculating router via a
point-to-point interface, no next hop IP address is required. If the
destination is a router connected to the calculating router via a
virtual link, the setting of the next hop should be deferred until
the calculation in Section 16.3.
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In the second case, the parent vertex is a network that directly
connects the calculating router to the destination router. The list
of next hops is then determined by examining the destination's
router-LSA. For each link in the router-LSA that points back to the
parent network, the link's Link Data field provides the IP address of
a next hop router. The outgoing interface to use can then be derived
from the next hop IP address (or it can be inherited from the parent
network).
16.2. Calculating the inter-area routes
The inter-area routes are calculated by examining summary-LSAs. If
the router has active attachments to multiple areas, only backbone
summary-LSAs are examined. Routers attached to a single area examine
that area's summary-LSAs. In either case, the summary-LSAs examined
below are all part of a single area's link state database (call it
Area A).
Summary-LSAs are originated by the area border routers. Each
summary-LSA in Area A is considered in turn. Remember that the
destination described by a summary-LSA is either a network (Type 3
summary-LSAs) or an AS boundary router (Type 4 summary-LSAs). For
each summary-LSA:
(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the the next
LSA.
(2) If the LSA was originated by the calculating router itself,
examine the next LSA.
(3) If it is a Type 3 summary-LSA, and the collection of
destinations described by the summary-LSA equals one of the
router's configured area address ranges (see Section 3.5),
and the particular area address range is active, then the
summary-LSA should be ignored. "Active" means that there
are one or more reachable (by intra-area paths) networks
contained in the area range.
(4) Else, call the destination described by the LSA N (for Type
3 summary-LSAs, N's address is obtained by masking the LSA's
Link State ID with the network/subnet mask contained in the
body of the LSA), and the area border originating the LSA
BR. Look up the routing table entry for BR having Area A as
its associated area. If no such entry exists for router BR
(i.e., BR is unreachable in Area A), do nothing with this
LSA and consider the next in the list. Else, this LSA
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describes an inter-area path to destination N, whose cost is
the distance to BR plus the cost specified in the LSA. Call
the cost of this inter-area path IAC.
(5) Next, look up the routing table entry for the destination N.
(If N is an AS boundary router, look up the "router" routing
table entry associated with Area A). If no entry exists for
N or if the entry's path type is "type 1 external" or "type
2 external", then install the inter-area path to N, with
associated area Area A, cost IAC, next hop equal to the list
of next hops to router BR, and Advertising router equal to
BR.
(6) Else, if the paths present in the table are intra-area
paths, do nothing with the LSA (intra-area paths are always
preferred).
(7) Else, the paths present in the routing table are also
inter-area paths. Install the new path through BR if it is
cheaper, overriding the paths in the routing table.
Otherwise, if the new path is the same cost, add it to the
list of paths that appear in the routing table entry.
16.3. Examining transit areas' summary-LSAs
This step is only performed by area border routers attached to one or
more non-backbone areas that are capable of carrying transit traffic
(i.e., "transit areas", or those areas whose TransitCapability
parameter has been set to TRUE in Step 2 of the Dijkstra algorithm
(see Section 16.1).
The purpose of the calculation below is to examine the transit areas
to see whether they provide any better (shorter) paths than the paths
previously calculated in Sections 16.1 and 16.2. Any paths found
that are better than or equal to previously discovered paths are
installed in the routing table.
The calculation proceeds as follows. All the transit areas' summary-
LSAs are examined in turn. Each such summary-LSA describes a route
through a transit area Area A to a Network N (N's address is obtained
by masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA) or in the case of a Type 4
summary-LSA, to an AS boundary router N. Suppose also that the
summary-LSA was originated by an area border router BR.
(1) If the cost advertised by the summary-LSA is LSInfinity, or
if the LSA's LS age is equal to MaxAge, then examine the
next LSA.
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(2) If the summary-LSA was originated by the calculating router
itself, examine the next LSA.
(3) Look up the routing table entry for N. (If N is an AS
boundary router, look up the "router" routing table entry
associated with the backbone area). If it does not exist, or
if the route type is other than intra-area or inter-area, or
if the area associated with the routing table entry is not
the backbone area, then examine the next LSA. In other
words, this calculation only updates backbone intra-area
routes found in Section 16.1 and inter-area routes found in
Section 16.2.
(4) Look up the routing table entry for the advertising router
BR associated with the Area A. If it is unreachable, examine
the next LSA. Otherwise, the cost to destination N is the
sum of the cost in BR's Area A routing table entry and the
cost advertised in the LSA. Call this cost IAC.
(5) If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC
is the same as N's current cost, add BR's list of next hops
to N's list of next hops. In any case, the area associated
with N's routing table entry must remain the backbone area,
and the path type (either intra-area or inter-area) must
also remain the same.
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. Area 1 (transit) . +
. . |
. +---+1 1+---+100 |
. |RT2|----------|RT4|=========|
. 1/+---+********* +---+ |
. /******* . |
. 1/*Virtual . |
1+---+/* Link . Net|work
=======|RT1|* . | N1
+---+\ . |
. \ . |
. \ . |
. 1\+---+1 1+---+20 |
. |RT3|----------|RT5|=========|
. +---+ +---+ |
. . |
........................ +
Figure 17: Routing through transit areas
It is important to note that the above calculation never makes
unreachable destinations reachable, but instead just potentially
finds better paths to already reachable destinations. The
calculation installs any better cost found into the routing table
entry, from which it may be readvertised in summary-LSAs to other
areas.
As an example of the calculation, consider the Autonomous System
pictured in Figure 17. There is a single non-backbone area (Area 1)
that physically divides the backbone into two separate pieces. To
maintain connectivity of the backbone, a virtual link has been
configured between routers RT1 and RT4. On the right side of the
figure, Network N1 belongs to the backbone. The dotted lines indicate
that there is a much shorter intra-area backbone path between router
RT5 and Network N1 (cost 20) than there is between Router RT4 and
Network N1 (cost 100). Both Router RT4 and Router RT5 will inject
summary-LSAs for Network N1 into Area 1.
After the shortest-path tree has been calculated for the backbone in
Section 16.1, Router RT1 (left end of the virtual link) will have
calculated a path through Router RT4 for all data traffic destined
for Network N1. However, since Router RT5 is so much closer to
Network N1, all routers internal to Area 1 (e.g., Routers RT2 and
RT3) will forward their Network N1 traffic towards Router RT5,
instead of RT4. And indeed, after examining Area 1's summary-LSAs by
the above calculation, Router RT1 will also forward Network N1
traffic towards RT5. Note that in this example the virtual link
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enables transit data traffic to be forwarded through Area 1, but the
actual path the transit data traffic takes does not follow the
virtual link. In other words, virtual links allow transit traffic to
be forwarded through an area, but do not dictate the precise path
that the traffic will take.
16.4. Calculating AS external routes
AS external routes are calculated by examining AS-external-LSAs.
Each of the AS-external-LSAs is considered in turn. Most AS-
external-LSAs describe routes to specific IP destinations. An AS-
external-LSA can also describe a default route for the Autonomous
System (Destination ID = DefaultDestination, network/subnet mask =
0x00000000). For each AS-external-LSA:
(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the next LSA.
(2) If the LSA was originated by the calculating router itself,
examine the next LSA.
(3) Call the destination described by the LSA N. N's address is
obtained by masking the LSA's Link State ID with the
network/subnet mask contained in the body of the LSA. Look
up the routing table entries (potentially one per attached
area) for the AS boundary router (ASBR) that originated the
LSA. If no entries exist for router ASBR (i.e., ASBR is
unreachable), do nothing with this LSA and consider the next
in the list.
Else, this LSA describes an AS external path to destination
N. Examine the forwarding address specified in the AS-
external-LSA. This indicates the IP address to which
packets for the destination should be forwarded.
If the forwarding address is set to 0.0.0.0, packets should
be sent to the ASBR itself. Among the multiple routing table
entries for the ASBR, select the preferred entry as follows.
If RFC1583Compatibility is set to "disabled", prune the set
of routing table entries for the ASBR as described in
Section 16.4.1. In any case, among the remaining routing
table entries, select the routing table entry with the least
cost; when there are multiple least cost routing table
entries the entry whose associated area has the largest OSPF
Area ID (when considered as an unsigned 32-bit integer) is
chosen.
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If the forwarding address is non-zero, look up the
forwarding address in the routing table.[24] The matching
routing table entry must specify an intra-area or inter-area
path; if no such path exists, do nothing with the LSA and
consider the next in the list.
(4) Let X be the cost specified by the preferred routing table
entry for the ASBR/forwarding address, and Y the cost
specified in the LSA. X is in terms of the link state
metric, and Y is a type 1 or 2 external metric.
(5) Look up the routing table entry for the destination N. If
no entry exists for N, install the AS external path to N,
with next hop equal to the list of next hops to the
forwarding address, and advertising router equal to ASBR.
If the external metric type is 1, then the path-type is set
to type 1 external and the cost is equal to X+Y. If the
external metric type is 2, the path-type is set to type 2
external, the link state component of the route's cost is X,
and the type 2 cost is Y.
(6) Compare the AS external path described by the LSA with the
existing paths in N's routing table entry, as follows. If
the new path is preferred, it replaces the present paths in
N's routing table entry. If the new path is of equal
preference, it is added to N's routing table entry's list of
paths.
(a) Intra-area and inter-area paths are always preferred
over AS external paths.
(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external
paths, the paths with the smallest advertised type 2
metric are always preferred.
(c) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
and RFC1583Compatibility is set to "disabled", select
the preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section16.4.1.
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(d) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
select the preferred path based on a least cost
comparison. Type 1 external paths are compared by
looking at the sum of the distance to the forwarding
address and the advertised type 1 metric (X+Y). Type 2
external paths advertising equal type 2 metrics are
compared by looking at the distance to the forwarding
addresses.
16.4.1. External path preferences
When multiple intra-AS paths are available to ASBRs/forwarding
addresses, the following rules indicate which paths are preferred.
These rules apply when the same ASBR is reachable through multiple
areas, or when trying to decide which of several AS-external-LSAs
should be preferred. In the former case the paths all terminate at
the same ASBR, while in the latter the paths terminate at separate
ASBRs/forwarding addresses. In either case, each path is represented
by a separate routing table entry as defined in Section 11.
This section only applies when RFC1583Compatibility is set to
"disabled".
The path preference rules, stated from highest to lowest preference,
are as follows. Note that as a result of these rules, there may still
be multiple paths of the highest preference. In this case, the path
to use must be determined based on cost, as described in Section16.4.
o Intra-area paths using non-backbone areas are always the
most preferred.
o Otherwise, intra-area backbone paths are preferred.
o Inter-area paths are the least preferred.
16.5. Incremental updates -- summary-LSAs
When a new summary-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination described
by the summary-LSA N (N's address is obtained by masking the LSA's
Link State ID with the network/subnet mask contained in the body of
the LSA), and let Area A be the area to which the LSA belongs. There
are then two separate cases:
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Case 1: Area A is the backbone and/or the router is not an area
border router.
In this case, the following calculations must be performed.
First, if there is presently an inter-area route to the
destination N, N's routing table entry is invalidated, saving the
entry's values for later comparisons. Then the calculation in
Section 16.2 is run again for the single destination N. In this
calculation, all of Area A's summary-LSAs that describe a route to
N are examined. In addition, if the router is an area border
router attached to one or more transit areas, the calculation in
Section 16.3 must be run again for the single destination. If the
results of these calculations have changed the cost/path to an AS
boundary router (as would be the case for a Type 4 summary-LSA) or
to any forwarding addresses, all AS- external-LSAs will have to be
reexamined by rerunning the calculation in Section 16.4.
Otherwise, if N is now newly unreachable, the calculation in
Section 16.4 must be rerun for the single destination N, in case
an alternate external route to N exists.
Case 2: Area A is a transit area and the router is an area border
router.
In this case, the following calculations must be performed.
First, if N's routing table entry presently contains one or more
inter-area paths that utilize the transit area Area A, these paths
should be removed. If this removes all paths from the routing
table entry, the entry should be invalidated. The entry's old
values should be saved for later comparisons. Next the calculation
in Section 16.3 must be run again for the single destination N. If
the results of this calculation have caused the cost to N to
increase, the complete routing table calculation must be rerun
starting with the Dijkstra algorithm specified in Section 16.1.
Otherwise, if the cost/path to an AS boundary router (as would be
the case for a Type 4 summary-LSA) or to any forwarding addresses
has changed, all AS-external-LSAs will have to be reexamined by
rerunning the calculation in Section 16.4. Otherwise, if N is now
newly unreachable, the calculation in Section 16.4 must be rerun
for the single destination N, in case an alternate external route
to N exists.
16.6. Incremental updates -- AS-external-LSAs
When a new AS-external-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination described
by the AS-external-LSA N. N's address is obtained by masking the
LSA's Link State ID with the network/subnet mask contained in the
body of the LSA. If there is already an intra- area or inter-area
route to the destination, no recalculation is necessary (internal
routes take precedence).
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Otherwise, the procedure in Section 16.4 will have to be performed,
but only for those AS-external-LSAs whose destination is N. Before
this procedure is performed, the present routing table entry for N
should be invalidated.
16.7. Events generated as a result of routing table changes
Changes to routing table entries sometimes cause the OSPF area border
routers to take additional actions. These routers need to act on the
following routing table changes:
o The cost or path type of a routing table entry has changed.
If the destination described by this entry is a Network or AS
boundary router, and this is not simply a change of AS external
routes, new summary-LSAs may have to be generated (potentially one
for each attached area, including the backbone). See Section12.4.3 for more information. If a previously advertised entry has
been deleted, or is no longer advertisable to a particular area,
the LSA must be flushed from the routing domain by setting its LS
age to MaxAge and reflooding (see Section 14.1).
o A routing table entry associated with a configured virtual
link has changed. The destination of such a routing table entry
is an area border router. The change indicates a modification to
the virtual link's cost or viability.
If the entry indicates that the area border router is newly
reachable, the corresponding virtual link is now operational. An
InterfaceUp event should be generated for the virtual link, which
will cause a virtual adjacency to begin to form (see Section10.3). At this time the virtual link's IP interface address and
the virtual neighbor's Neighbor IP address are also calculated.
If the entry indicates that the area border router is no longer
reachable, the virtual link and its associated adjacency should be
destroyed. This means an InterfaceDown event should be generated
for the associated virtual link.
If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router-LSA for the backbone
must be originated. This in turn may cause further routing table
changes.
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RFC 2178 OSPF Version 2 July 199716.8. Equal-cost multipath
The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to calculate
the routing table, and in the definition of the routing table
structure.
Each one of the multiple routes will be of the same type (intra-area,
inter-area, type 1 external or type 2 external), cost, and will have
the same associated area. However, each route specifies a separate
next hop and Advertising router.
There is no requirement that a router running OSPF keep track of all
possible equal-cost routes to a destination. An implementation may
choose to keep only a fixed number of routes to any given
destination. This does not affect any of the algorithms presented in
this specification.
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Footnotes
[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.
[2]It is possible for all of a router's interfaces to be unnumbered
point-to-point links. In this case, an IP address must be assigned
to the router. This address will then be advertised in the router's
router-LSA as a host route.
[3]Note that in these cases both interfaces, the non-virtual and the
virtual, would have the same IP address.
[4]Note that no host route is generated for, and no IP packets can be
addressed to, interfaces to unnumbered point-to-point networks. This
is regardless of such an interface's state.
[5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
RT1, and the Backup Designated Router RT2. If Router RT1 crashes (or
maybe its interface to the network dies), the other routers on the
network will detect RT1's absence within RouterDeadInterval seconds.
All routers may not detect this at precisely the same time; the
routers that detect RT1's absence before RT2 does will, for a time,
select RT2 to be both Designated Router and Backup Designated Router.
When RT2 detects that RT1 is gone it will move itself to Designated
Router. At this time, the remaining router having highest Router
Priority will be selected as Backup Designated Router.
[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
neighbor on virtual links is indicated by the routing table
calculation. However, in both these cases, the Hello Protocol is
still used. This ensures that communication between the neighbors is
bidirectional, and that each of the neighbors has a functioning
routing protocol layer.
[7]When the identity of the Designated Router is changing, it may be
quite common for a neighbor in this state to send the router a
Database Description packet; this means that there is some momentary
disagreement on the Designated Router's identity.
[8]Note that it is possible for a router to resynchronize any of its
fully established adjacencies by setting the adjacency's state back
to ExStart. This will cause the other end of the adjacency to
process a SeqNumberMismatch event, and therefore to also go back to
ExStart state.
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[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number) to some
router's Router ID.
[10]"Discard" entries are necessary to ensure that route
summarization at area boundaries will not cause packet looping.
[11]It is assumed that, for two different address ranges matching the
destination, one range is more specific than the other. Non-
contiguous subnet masks can be configured to violate this assumption.
Such subnet mask configurations cannot be handled by the OSPF
protocol.
[12]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
LSA instance as it is flooded throughout the routing domain. If two
LSAs differ by more than this, they are assumed to be different
instances of the same LSA. This can occur when a router restarts and
loses track of the LSA's previous LS sequence number. See Section13.4 for more details.
[13]When two LSAs have different LS checksums, they are assumed to be
separate instances. This can occur when a router restarts, and loses
track of the LSA's previous LS sequence number. In the case where
the two LSAs have the same LS sequence number, it is not possible to
determine which LSA is actually newer. However, if the wrong LSA is
accepted as newer, the originating router will simply originate
another instance. See Section 13.4 for further details.
[14]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network-LSA must be found based solely on its Link State ID.
The lookup in this case is still well defined, since no two network-
LSAs can have the same Link State ID.
[15]This is the way RFC 1583 specified point-to-point representation.
It has three advantages: a) it does not require allocating a subnet
to the point-to-point link, b) it tends to bias the routing so that
packets destined for the point-to-point interface will actually be
received over the interface (which is useful for diagnostic purposes)
and c) it allows network bootstrapping of a neighbor, without
requiring that the bootstrap program contain an OSPF implementation.
[16]This is the more traditional point-to-point representation used
by protocols such as RIP.
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RFC 2178 OSPF Version 2 July 1997
[17]This clause covers the case: Inter-area routes are not summarized
to the backbone. This is because inter-area routes are always
associated with the backbone area.
[18]This clause is only invoked when a non-backbone Area A supports
transit data traffic (i.e., has TransitCapability set to TRUE). For
example, in the area configuration of Figure 6, Area 2 can support
transit traffic due to the configured virtual link between Routers
RT10 and RT11. As a result, Router RT11 need only originate a single
summary-LSA into Area 2 (having the collapsed destination N9-N11,H1),
since all of Router RT11's other eligible routes have next hops
belonging to Area 2 itself (and as such only need be advertised by
other area border routers; in this case, Routers RT10 and RT7).
[19]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree for only
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of a single area's
shortest path tree [Ref1]. However, these algorithms are beyond the
scope of this specification.
[20]This is how the Link state request list is emptied, which
eventually causes the neighbor state to transition to Full. See
Section 10.9 for more details.
[21]It should be a relatively rare occurrence for an LSA's LS age to
reach MaxAge in this fashion. Usually, the LSA will be replaced by a
more recent instance before it ages out.
[22]Strictly speaking, because of equal-cost multipath, the algorithm
does not create a tree. We continue to use the "tree" terminology
because that is what occurs most often in the existing literature.
[23]Note that the presence of any link back to V is sufficient; it
need not be the matching half of the link under consideration from V
to W. This is enough to ensure that, before data traffic flows
between a pair of neighboring routers, their link state databases
will be synchronized.
[24]When the forwarding address is non-zero, it should point to a
router belonging to another Autonomous System. See Section 12.4.4
for more details.
Moy Standards Track [Page 157]

RFC 2178 OSPF Version 2 July 1997A. OSPF data formats
This appendix describes the format of OSPF protocol packets and OSPF
LSAs. The OSPF protocol runs directly over the IP network layer.
Before any data formats are described, the details of the OSPF
encapsulation are explained.
Next the OSPF Options field is described. This field describes
various capabilities that may or may not be supported by pieces of
the OSPF routing domain. The OSPF Options field is contained in OSPF
Hello packets, Database Description packets and in OSPF LSAs.
OSPF packet formats are detailed in Section A.3. A description of
OSPF LSAs appears in Section A.4.
A.1 Encapsulation of OSPF packets
OSPF runs directly over the Internet Protocol's network layer. OSPF
packets are therefore encapsulated solely by IP and local data-link
headers.
OSPF does not define a way to fragment its protocol packets, and
depends on IP fragmentation when transmitting packets larger than the
network MTU. If necessary, the length of OSPF packets can be up to
65,535 bytes (including the IP header). The OSPF packet types that
are likely to be large (Database Description Packets, Link State
Request, Link State Update, and Link State Acknowledgment packets)
can usually be split into several separate protocol packets, without
loss of functionality. This is recommended; IP fragmentation should
be avoided whenever possible. Using this reasoning, an attempt should
be made to limit the sizes of OSPF packets sent over virtual links to
576 bytes unless Path MTU Discovery is being performed (see [Ref22]).
The other important features of OSPF's IP encapsulation are:
o Use of IP multicast. Some OSPF messages are multicast, when
sent over broadcast networks. Two distinct IP multicast addresses
are used. Packets sent to these multicast addresses should never
be forwarded; they are meant to travel a single hop only. To
ensure that these packets will not travel multiple hops, their IP
TTL must be set to 1.
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RFC 2178 OSPF Version 2 July 1997
AllSPFRouters
This multicast address has been assigned the value 224.0.0.5. All
routers running OSPF should be prepared to receive packets sent to
this address. Hello packets are always sent to this destination.
Also, certain OSPF protocol packets are sent to this address
during the flooding procedure.
AllDRouters
This multicast address has been assigned the value 224.0.0.6. Both
the Designated Router and Backup Designated Router must be
prepared to receive packets destined to this address. Certain
OSPF protocol packets are sent to this address during the flooding
procedure.
o OSPF is IP protocol number 89. This number has been registered
with the Network Information Center. IP protocol number
assignments are documented in [Ref11].
o All OSPF routing protocol packets are sent using the normal
service TOS value of binary 0000 defined in [Ref12].
o Routing protocol packets are sent with IP precedence set to
Internetwork Control. OSPF protocol packets should be given
precedence over regular IP data traffic, in both sending and
receiving. Setting the IP precedence field in the IP header to
Internetwork Control [Ref5] may help implement this objective.
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RFC 2178 OSPF Version 2 July 1997A.2 The Options field
The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all LSAs. The Options field enables OSPF
routers to support (or not support) optional capabilities, and to
communicate their capability level to other OSPF routers. Through
this mechanism routers of differing capabilities can be mixed within
an OSPF routing domain.
When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain LSAs to a neighbor because
of its reduced functionality. Lastly, listing capabilities in LSAs
allows routers to forward traffic around reduced functionality
routers, by excluding them from parts of the routing table
calculation.
Five bits of the OSPF Options field have been assigned, although only
one (the E-bit) is described completely by this memo. Each bit is
described briefly below. Routers should reset (i.e. clear)
unrecognized bits in the Options field when sending Hello packets or
Database Description packets and when originating LSAs. Conversely,
routers encountering unrecognized Option bits in received Hello
Packets, Database Description packets or LSAs should ignore the
capability and process the packet/LSA normally.
+------------------------------------+
| * | * | DC | EA | N/P | MC | E | * |
+------------------------------------+
The Options field
E-bit
This bit describes the way AS-external-LSAs are flooded, as
described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.
MC-bit
This bit describes whether IP multicast datagrams are forwarded
according to the specifications in [Ref18].
N/P-bit
This bit describes the handling of Type-7 LSAs, as specified in
[Ref19].
EA-bit
This bit describes the router's willingness to receive and
forward External-Attributes-LSAs, as specified in [Ref20].
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RFC 2178 OSPF Version 2 July 1997
DC-bit
This bit describes the router's handling of demand circuits, as
specified in [Ref21].
A.3 OSPF Packet Formats
There are five distinct OSPF packet types. All OSPF packet types
begin with a standard 24 byte header. This header is described
first. Each packet type is then described in a succeeding section.
In these sections each packet's division into fields is displayed,
and then the field definitions are enumerated.
All OSPF packet types (other than the OSPF Hello packets) deal with
lists of LSAs. For example, Link State Update packets implement the
flooding of LSAs throughout the OSPF routing domain. Because of
this, OSPF protocol packets cannot be parsed unless the format of
LSAs is also understood. The format of LSAs is described in Section
A.4.
The receive processing of OSPF packets is detailed in Section 8.2.
The sending of OSPF packets is explained in Section 8.1.
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RFC 2178 OSPF Version 2 July 1997
Area ID
A 32 bit number identifying the area that this packet belongs
to. All OSPF packets are associated with a single area. Most
travel a single hop only. Packets travelling over a virtual
link are labelled with the backbone Area ID of 0.0.0.0.
Checksum
The standard IP checksum of the entire contents of the packet,
starting with the OSPF packet header but excluding the 64-bit
authentication field. This checksum is calculated as the 16-bit
one's complement of the one's complement sum of all the 16-bit
words in the packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit words, the
packet is padded with a byte of zero before checksumming. The
checksum is considered to be part of the packet authentication
procedure; for some authentication types the checksum
calculation is omitted.
AuType
Identifies the authentication procedure to be used for the
packet. Authentication is discussed in Appendix D of the
specification. Consult Appendix D for a list of the currently
defined authentication types.
Authentication
A 64-bit field for use by the authentication scheme. See
Appendix D for details.
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RFC 2178 OSPF Version 2 July 1997
Network mask
The network mask associated with this interface. For example,
if the interface is to a class B network whose third byte is
used for subnetting, the network mask is 0xffffff00.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
HelloInterval
The number of seconds between this router's Hello packets.
Rtr Pri
This router's Router Priority. Used in (Backup) Designated
Router election. If set to 0, the router will be ineligible to
become (Backup) Designated Router.
RouterDeadInterval
The number of seconds before declaring a silent router down.
Designated Router
The identity of the Designated Router for this network, in the
view of the sending router. The Designated Router is identified
here by its IP interface address on the network. Set to 0.0.0.0
if there is no Designated Router.
Backup Designated Router
The identity of the Backup Designated Router for this network,
in the view of the sending router. The Backup Designated Router
is identified here by its IP interface address on the network.
Set to 0.0.0.0 if there is no Backup Designated Router.
Neighbor
The Router IDs of each router from whom valid Hello packets have
been seen recently on the network. Recently means in the last
RouterDeadInterval seconds.
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RFC 2178 OSPF Version 2 July 1997
Description Packets is documented in Section 10.8. The reception of
Database Description packets is documented in Section 10.6.
Interface MTU
The size in bytes of the largest IP datagram that can be sent out
the associated interface, without fragmentation. The MTUs of
common Internet link types can be found in Table 7-1 of [Ref22].
Interface MTU should be set to 0 in Database Description packets
sent over virtual links.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
I-bit
The Init bit. When set to 1, this packet is the first in the
sequence of Database Description Packets.
M-bit
The More bit. When set to 1, it indicates that more Database
Description Packets are to follow.
MS-bit
The Master/Slave bit. When set to 1, it indicates that the router
is the master during the Database Exchange process. Otherwise,
the router is the slave.
DD sequence number
Used to sequence the collection of Database Description Packets.
The initial value (indicated by the Init bit being set) should be
unique. The DD sequence number then increments until the complete
database description has been sent.
The rest of the packet consists of a (possibly partial) list of the
link-state database's pieces. Each LSA in the database is described
by its LSA header. The LSA header is documented in Section A.4.1. It
contains all the information required to uniquely identify both the
LSA and the LSA's current instance.
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RFC 2178 OSPF Version 2 July 1997A.3.4 The Link State Request packet
Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring router, a router may
find that parts of its link-state database are out-of-date. The Link
State Request packet is used to request the pieces of the neighbor's
database that are more up-to-date. Multiple Link State Request
packets may need to be used.
A router that sends a Link State Request packet has in mind the
precise instance of the database pieces it is requesting. Each
instance is defined by its LS sequence number, LS checksum, and LS
age, although these fields are not specified in the Link State
Request Packet itself. The router may receive even more recent
instances in response.
The sending of Link State Request packets is documented in Section10.9. The reception of Link State Request packets is documented in
Section 10.7.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 3 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Each LSA requested is specified by its LS type, Link State ID, and
Advertising Router. This uniquely identifies the LSA, but not its
instance. Link State Request packets are understood to be requests
for the most recent instance (whatever that might be).
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RFC 2178 OSPF Version 2 July 1997A.3.5 The Link State Update packet
Link State Update packets are OSPF packet type 4. These packets
implement the flooding of LSAs. Each Link State Update packet
carries a collection of LSAs one hop further from their origin.
Several LSAs may be included in a single packet.
Link State Update packets are multicast on those physical networks
that support multicast/broadcast. In order to make the flooding
procedure reliable, flooded LSAs are acknowledged in Link State
Acknowledgment packets. If retransmission of certain LSAs is
necessary, the retransmitted LSAs are always carried by unicast Link
State Update packets. For more information on the reliable flooding
of LSAs, consult Section 13.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 4 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # LSAs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- +-+
| LSAs |
+- +-+
| ... |
# LSAs
The number of LSAs included in this update.
The body of the Link State Update packet consists of a list of LSAs.
Each LSA begins with a common 20 byte header, described in Section
A.4.1. Detailed formats of the different types of LSAs are described
in Section A.4.
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RFC 2178 OSPF Version 2 July 1997
Each acknowledged LSA is described by its LSA header. The LSA header
is documented in Section A.4.1. It contains all the information
required to uniquely identify both the LSA and the LSA's current
instance.
A.4 LSA formats
This memo defines five distinct types of LSAs. Each LSA begins with
a standard 20 byte LSA header. This header is explained in Section
A.4.1. Succeeding sections then diagram the separate LSA types.
Each LSA describes a piece of the OSPF routing domain. Every router
originates a router-LSA. In addition, whenever the router is elected
Designated Router, it originates a network-LSA. Other types of LSAs
may also be originated (see Section 12.4). All LSAs are then flooded
throughout the OSPF routing domain. The flooding algorithm is
reliable, ensuring that all routers have the same collection of LSAs.
(See Section 13 for more information concerning the flooding
algorithm). This collection of LSAs is called the link-state
database.
From the link state database, each router constructs a shortest path
tree with itself as root. This yields a routing table (see Section11). For the details of the routing table build process, see Section16.
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RFC 2178 OSPF Version 2 July 1997
Link State ID
This field identifies the portion of the internet environment
that is being described by the LSA. The contents of this field
depend on the LSA's LS type. For example, in network-LSAs the
Link State ID is set to the IP interface address of the
network's Designated Router (from which the network's IP address
can be derived). The Link State ID is further discussed in
Section 12.1.4.
Advertising Router
The Router ID of the router that originated the LSA. For
example, in network-LSAs this field is equal to the Router ID of
the network's Designated Router.
LS sequence number
Detects old or duplicate LSAs. Successive instances of an LSA
are given successive LS sequence numbers. See Section 12.1.6
for more details.
LS checksum
The Fletcher checksum of the complete contents of the LSA,
including the LSA header but excluding the LS age field. See
Section 12.1.7 for more details.
length
The length in bytes of the LSA. This includes the 20 byte LSA
header.
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RFC 2178 OSPF Version 2 July 1997
bit E
When set, the router is an AS boundary router (E is for external).
bit B
When set, the router is an area border router (B is for border).
# links
The number of router links described in this LSA. This must be
the total collection of router links (i.e., interfaces) to the
area.
The following fields are used to describe each router link (i.e.,
interface). Each router link is typed (see the below Type field).
The Type field indicates the kind of link being described. It may be
a link to a transit network, to another router or to a stub network.
The values of all the other fields describing a router link depend on
the link's Type. For example, each link has an associated 32-bit
Link Data field. For links to stub networks this field specifies the
network's IP address mask. For other link types the Link Data field
specifies the router interface's IP address.
Type
A quick description of the router link. One of the following.
Note that host routes are classified as links to stub networks
with network mask of 0xffffffff.
Type Description
__________________________________________________
1 Point-to-point connection to another router
2 Connection to a transit network
3 Connection to a stub network
4 Virtual link
Link ID
Identifies the object that this router link connects to. Value
depends on the link's Type. When connecting to an object that
also originates an LSA (i.e., another router or a transit
network) the Link ID is equal to the neighboring LSA's Link
State ID. This provides the key for looking up the neighboring
LSA in the link state database during the routing table
calculation. See Section 12.2 for more details.
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RFC 2178 OSPF Version 2 July 1997
Type Link ID
______________________________________
1 Neighboring router's Router ID
2 IP address of Designated Router
3 IP network/subnet number
4 Neighboring router's Router ID
Link Data
Value again depends on the link's Type field. For connections to
stub networks, Link Data specifies the network's IP address
mask. For unnumbered point-to-point connections, it specifies
the interface's MIB-II [Ref8] ifIndex value. For the other link
types it specifies the router interface's IP address. This
latter piece of information is needed during the routing table
build process, when calculating the IP address of the next hop.
See Section 16.1.1 for more details.
# TOS
The number of different TOS metrics given for this link, not
counting the required link metric (referred to as the TOS 0
metric in [Ref9]). For example, if no additional TOS metrics
are given, this field is set to 0.
metric
The cost of using this router link.
Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). Within each link, and for each desired TOS,
TOS TOS-specific link information may be encoded as follows:
TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF LSAs is described in Section 12.3.
TOS metric
TOS-specific metric information.
Moy Standards Track [Page 178]

RFC 2178 OSPF Version 2 July 1997A.4.3 Network-LSAs
Network-LSAs are the Type 2 LSAs. A network-LSA is originated for
each broadcast and NBMA network in the area which supports two or
more routers. The network-LSA is originated by the network's
Designated Router. The LSA describes all routers attached to the
network, including the Designated Router itself. The LSA's Link
State ID field lists the IP interface address of the Designated
Router.
The distance from the network to all attached routers is zero. This
is why metric fields need not be specified in the network-LSA. For
details concerning the construction of network-LSAs, see Section12.4.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network Mask
The IP address mask for the network. For example, a class A
network would have the mask 0xff000000.
Attached Router
The Router IDs of each of the routers attached to the network.
Actually, only those routers that are fully adjacent to the
Designated Router are listed. The Designated Router includes
itself in this list. The number of routers included can be
deduced from the LSA header's length field.
Moy Standards Track [Page 179]

RFC 2178 OSPF Version 2 July 1997A.4.4 Summary-LSAs
Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated by
area border routers. Summary-LSAs describe inter-area destinations.
For details concerning the construction of summary-LSAs, see Section12.4.3.
Type 3 summary-LSAs are used when the destination is an IP network.
In this case the LSA's Link State ID field is an IP network number
(if necessary, the Link State ID can also have one or more of the
network's "host" bits set; see Appendix E for details). When the
destination is an AS boundary router, a Type 4 summary-LSA is used,
and the Link State ID field is the AS boundary router's OSPF Router
ID. (To see why it is necessary to advertise the location of each
ASBR, consult Section 16.4.) Other than the difference in the Link
State ID field, the format of Type 3 and 4 summary-LSAs is identical.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
For stub areas, Type 3 summary-LSAs can also be used to describe a
(per-area) default route. Default summary routes are used in stub
areas instead of flooding a complete set of external routes. When
describing a default summary route, the summary-LSA's Link State ID
is always set to DefaultDestination (0.0.0.0) and the Network Mask is
set to 0.0.0.0.
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RFC 2178 OSPF Version 2 July 1997
Network Mask
For Type 3 summary-LSAs, this indicates the destination network's
IP address mask. For example, when advertising the location of a
class A network the value 0xff000000 would be used. This field is
not meaningful and must be zero for Type 4 summary-LSAs.
metric
The cost of this route. Expressed in the same units as the
interface costs in the router-LSAs.
Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). For each desired TOS, TOS-specific
information is encoded as follows:
TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF LSAs is described in Section 12.3.
TOS metric
TOS-specific metric information.
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RFC 2178 OSPF Version 2 July 1997
Network Mask
The IP address mask for the advertised destination. For
example, when advertising a class A network the mask 0xff000000
would be used.
bit E
The type of external metric. If bit E is set, the metric
specified is a Type 2 external metric. This means the metric is
considered larger than any link state path. If bit E is zero,
the specified metric is a Type 1 external metric. This means
that it is expressed in the same units as the link state metric
(i.e., the same units as interface cost).
metric
The cost of this route. Interpretation depends on the external
type indication (bit E above).
Forwarding address
Data traffic for the advertised destination will be forwarded to
this address. If the Forwarding address is set to 0.0.0.0, data
traffic will be forwarded instead to the LSA's originator (i.e.,
the responsible AS boundary router).
External Route Tag
A 32-bit field attached to each external route. This is not
used by the OSPF protocol itself. It may be used to communicate
information between AS boundary routers; the precise nature of
such information is outside the scope of this specification.
Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). For each desired TOS, TOS-specific
information is encoded as follows:
TOS The Type of Service that the following fields concern. The
encoding of TOS in OSPF LSAs is described in Section 12.3.
bit E
For backward-compatibility with [Ref9].
TOS metric
TOS-specific metric information.
Forwarding address
For backward-compatibility with [Ref9].
External Route Tag
For backward-compatibility with [Ref9].
Moy Standards Track [Page 183]

RFC 2178 OSPF Version 2 July 1997B. Architectural Constants
Several OSPF protocol parameters have fixed architectural values.
These parameters have been referred to in the text by names such as
LSRefreshTime. The same naming convention is used for the
configurable protocol parameters. They are defined in Appendix C.
The name of each architectural constant follows, together with its
value and a short description of its function.
LSRefreshTime
The maximum time between distinct originations of any particular
LSA. If the LS age field of one of the router's self-originated
LSAs reaches the value LSRefreshTime, a new instance of the LSA is
originated, even though the contents of the LSA (apart from the
LSA header) will be the same. The value of LSRefreshTime is set
to 30 minutes.
MinLSInterval
The minimum time between distinct originations of any particular
LSA. The value of MinLSInterval is set to 5 seconds.
MinLSArrival
For any particular LSA, the minimum time that must elapse
between reception of new LSA instances during flooding. LSA
instances received at higher frequencies are discarded. The value
of MinLSArrival is set to 1 second.
MaxAge
The maximum age that an LSA can attain. When an LSA's LS age field
reaches MaxAge, it is reflooded in an attempt to flush the LSA
from the routing domain (See Section 14). LSAs of age MaxAge are
not used in the routing table calculation. The value of MaxAge is
set to 1 hour.
CheckAge
When the age of an LSA in the link state database hits a multiple
of CheckAge, the LSA's checksum is verified. An incorrect
checksum at this time indicates a serious error. The value of
CheckAge is set to 5 minutes.
MaxAgeDiff
The maximum time dispersion that can occur, as an LSA is flooded
throughout the AS. Most of this time is accounted for by the LSAs
sitting on router output queues (and therefore not aging) during
the flooding process. The value of MaxAgeDiff is set to 15
minutes.
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LSInfinity
The metric value indicating that the destination described by an
LSA is unreachable. Used in summary-LSAs and AS-external-LSAs as
an alternative to premature aging (see Section 14.1). It is
defined to be the 24-bit binary value of all ones: 0xffffff.
DefaultDestination
The Destination ID that indicates the default route. This route
is used when no other matching routing table entry can be found.
The default destination can only be advertised in AS-external-
LSAs and in stub areas' type 3 summary-LSAs. Its value is the IP
address 0.0.0.0. Its associated Network Mask is also always
0.0.0.0.
InitialSequenceNumber
The value used for LS Sequence Number when originating the first
instance of any LSA. Its value is the signed 32-bit integer
0x80000001.
MaxSequenceNumber
The maximum value that LS Sequence Number can attain. Its value
is the signed 32-bit integer 0x7fffffff.
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RFC 2178 OSPF Version 2 July 1997C. Configurable Constants
The OSPF protocol has quite a few configurable parameters. These
parameters are listed below. They are grouped into general
functional categories (area parameters, interface parameters, etc.).
Sample values are given for some of the parameters.
Some parameter settings need to be consistent among groups of
routers. For example, all routers in an area must agree on that
area's parameters, and all routers attached to a network must agree
on that network's IP network number and mask.
Some parameters may be determined by router algorithms outside of
this specification (e.g., the address of a host connected to the
router via a SLIP line). From OSPF's point of view, these items are
still configurable.
C.1 Global parameters
In general, a separate copy of the OSPF protocol is run for each
area. Because of this, most configuration parameters are defined on
a per-area basis. The few global configuration parameters are listed
below.
Router ID
This is a 32-bit number that uniquely identifies the router in
the Autonomous System. One algorithm for Router ID assignment is
to choose the largest or smallest IP address assigned to the
router. If a router's OSPF Router ID is changed, the router's
OSPF software should be restarted before the new Router ID takes
effect. Before restarting in order to change its Router ID, the
router should flush its self-originated LSAs from the routing
domain (see Section 14.1), or they will persist for up to MaxAge
minutes.
RFC1583Compatibility
Controls the preference rules used in Section 16.4 when choosing
among multiple AS-external-LSAs advertising the same destination.
When set to "enabled", the preference rules remain those
specified by RFC 1583 ([Ref9]). When set to "disabled", the
preference rules are those stated in Section 16.4.1, which
prevent routing loops when AS- external-LSAs for the same
destination have been originated from different areas (see
Section G.7). Set to "enabled" by default.
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RFC 2178 OSPF Version 2 July 1997
In order to minimize the chance of routing loops, all OSPF
routers in an OSPF routing domain should have
RFC1583Compatibility set identically. When there are routers
present that have not been updated with the functionality
specified in Section 16.4.1 of this memo, all routers should have
RFC1583Compatibility set to "enabled". Otherwise, all routers
should have RFC1583Compatibility set to "disabled", preventing
all routing loops.
C.2 Area parameters
All routers belonging to an area must agree on that area's
configuration. Disagreements between two routers will lead to an
inability for adjacencies to form between them, with a resulting
hindrance to the flow of routing protocol and data traffic. The
following items must be configured for an area:
Area ID
This is a 32-bit number that identifies the area. The Area ID of
0.0.0.0 is reserved for the backbone. If the area represents a
subnetted network, the IP network number of the subnetted network
may be used for the Area ID.
List of address ranges
An OSPF area is defined as a list of address ranges. Each address
range consists of the following items:
[IP address, mask]
Describes the collection of IP addresses contained in the
address range. Networks and hosts are assigned to an area
depending on whether their addresses fall into one of the
area's defining address ranges. Routers are viewed as
belonging to multiple areas, depending on their attached
networks' area membership.
Status Set to either Advertise or DoNotAdvertise. Routing
information is condensed at area boundaries. External to the
area, at most a single route is advertised (via a summary-
LSA) for each address range. The route is advertised if and
only if the address range's Status is set to Advertise.
Unadvertised ranges allow the existence of certain networks
to be intentionally hidden from other areas. Status is set to
Advertise by default.
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As an example, suppose an IP subnetted network is to be its
own OSPF area. The area would be configured as a single
address range, whose IP address is the address of the
subnetted network, and whose mask is the natural class A, B,
or C address mask. A single route would be advertised
external to the area, describing the entire subnetted
network.
ExternalRoutingCapability
Whether AS-external-LSAs will be flooded into/throughout the
area. If AS-external-LSAs are excluded from the area, the
area is called a "stub". Internal to stub areas, routing to
external destinations will be based solely on a default
summary route. The backbone cannot be configured as a stub
area. Also, virtual links cannot be configured through stub
areas. For more information, see Section 3.6.
StubDefaultCost
If the area has been configured as a stub area, and the
router itself is an area border router, then the
StubDefaultCost indicates the cost of the default summary-LSA
that the router should advertise into the area.
C.3 Router interface parameters
Some of the configurable router interface parameters (such as IP
interface address and subnet mask) actually imply properties of the
attached networks, and therefore must be consistent across all the
routers attached to that network. The parameters that must be
configured for a router interface are:
IP interface address
The IP protocol address for this interface. This uniquely
identifies the router over the entire internet. An IP address is
not required on point-to-point networks. Such a point-to-point
network is called "unnumbered".
IP interface mask
Also referred to as the subnet/network mask, this indicates the
portion of the IP interface address that identifies the attached
network. Masking the IP interface address with the IP interface
mask yields the IP network number of the attached network. On
point-to-point networks and virtual links, the IP interface mask
is not defined. On these networks, the link itself is not
assigned an IP network number, and so the addresses of each side
of the link are assigned independently, if they are assigned at
all.
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RFC 2178 OSPF Version 2 July 1997
Area ID
The OSPF area to which the attached network belongs.
Interface output cost
The cost of sending a packet on the interface, expressed in the
link state metric. This is advertised as the link cost for this
interface in the router's router-LSA. The interface output cost
must always be greater than 0.
RxmtInterval
The number of seconds between LSA retransmissions, for
adjacencies belonging to this interface. Also used when
retransmitting Database Description and Link State Request
Packets. This should be well over the expected round-trip delay
between any two routers on the attached network. The setting of
this value should be conservative or needless retransmissions
will result. Sample value for a local area network: 5 seconds.
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update Packet over this interface. LSAs contained in the update
packet must have their age incremented by this amount before
transmission. This value should take into account the
transmission and propagation delays of the interface. It must be
greater than 0. Sample value for a local area network: 1 second.
Router Priority
An 8-bit unsigned integer. When two routers attached to a network
both attempt to become Designated Router, the one with the
highest Router Priority takes precedence. If there is still a
tie, the router with the highest Router ID takes precedence. A
router whose Router Priority is set to 0 is ineligible to become
Designated Router on the attached network. Router Priority is
only configured for interfaces to broadcast and NBMA networks.
HelloInterval
The length of time, in seconds, between the Hello Packets that
the router sends on the interface. This value is advertised in
the router's Hello Packets. It must be the same for all routers
attached to a common network. The smaller the HelloInterval, the
faster topological changes will be detected; however, more OSPF
routing protocol traffic will ensue. Sample value for a X.25 PDN
network: 30 seconds. Sample value for a local area network: 10
seconds.
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RFC 2178 OSPF Version 2 July 1997
RouterDeadInterval
After ceasing to hear a router's Hello Packets, the number of
seconds before its neighbors declare the router down. This is
also advertised in the router's Hello Packets in their
RouterDeadInterval field. This should be some multiple of the
HelloInterval (say 4). This value again must be the same for all
routers attached to a common network.
AuType
Identifies the authentication procedure to be used on the
attached network. This value must be the same for all routers
attached to the network. See Appendix D for a discussion of the
defined authentication types.
Authentication key
This configured data allows the authentication procedure to
verify OSPF protocol packets received over the interface. For
example, if the AuType indicates simple password, the
Authentication key would be a clear 64-bit password.
Authentication keys associated with the other OSPF authentication
types are discussed in Appendix D.
C.4 Virtual link parameters
Virtual links are used to restore/increase connectivity of the
backbone. Virtual links may be configured between any pair of area
border routers having interfaces to a common (non-backbone) area.
The virtual link appears as an unnumbered point-to-point link in the
graph for the backbone. The virtual link must be configured in both
of the area border routers.
A virtual link appears in router-LSAs (for the backbone) as if it
were a separate router interface to the backbone. As such, it has
all of the parameters associated with a router interface (see Section
C.3). Although a virtual link acts like an unnumbered point-to-point
link, it does have an associated IP interface address. This address
is used as the IP source in OSPF protocol packets it sends along the
virtual link, and is set dynamically during the routing table build
process. Interface output cost is also set dynamically on virtual
links to be the cost of the intra-area path between the two routers.
The parameter RxmtInterval must be configured, and should be well
over the expected round-trip delay between the two routers. This may
be hard to estimate for a virtual link; it is better to err on the
side of making it too large. Router Priority is not used on virtual
links.
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A virtual link is defined by the following two configurable
parameters: the Router ID of the virtual link's other endpoint, and
the (non-backbone) area through which the virtual link runs (referred
to as the virtual link's Transit area). Virtual links cannot be
configured through stub areas.
C.5 NBMA network parameters
OSPF treats an NBMA network much like it treats a broadcast network.
Since there may be many routers attached to the network, a Designated
Router is selected for the network. This Designated Router then
originates a network-LSA, which lists all routers attached to the
NBMA network.
However, due to the lack of broadcast capabilities, it may be
necessary to use configuration parameters in the Designated Router
selection. These parameters will only need to be configured in those
routers that are themselves eligible to become Designated Router
(i.e., those router's whose Router Priority for the network is non-
zero), and then only if no automatic procedure for discovering
neighbors exists:
List of all other attached routers
The list of all other routers attached to the NBMA network. Each
router is listed by its IP interface address on the network.
Also, for each router listed, that router's eligibility to become
Designated Router must be defined. When an interface to a NBMA
network comes up, the router sends Hello Packets only to those
neighbors eligible to become Designated Router, until the
identity of the Designated Router is discovered.
PollInterval
If a neighboring router has become inactive (Hello Packets have
not been seen for RouterDeadInterval seconds), it may still be
necessary to send Hello Packets to the dead neighbor. These
Hello Packets will be sent at the reduced rate PollInterval,
which should be much larger than HelloInterval. Sample value for
a PDN X.25 network: 2 minutes.
C.6 Point-to-MultiPoint network parameters
On Point-to-MultiPoint networks, it may be necessary to configure the
set of neighbors that are directly reachable over the Point-to-
MultiPoint network. Each neighbor is identified by its IP address on
the Point-to-MultiPoint network. Designated Routers are not elected
on Point-to-MultiPoint networks, so the Designated Router eligibility
of configured neighbors is undefined.
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RFC 2178 OSPF Version 2 July 1997
Alternatively, neighbors on Point-to-MultiPoint networks may be
dynamically discovered by lower-level protocols such as Inverse ARP
([Ref14]).
C.7 Host route parameters
Host routes are advertised in router-LSAs as stub networks with mask
0xffffffff. They indicate either router interfaces to point-to-point
networks, looped router interfaces, or IP hosts that are directly
connected to the router (e.g., via a SLIP line). For each host
directly connected to the router, the following items must be
configured:
Host IP address
The IP address of the host.
Cost of link to host
The cost of sending a packet to the host, in terms of the link
state metric. However, since the host probably has only a single
connection to the internet, the actual configured cost in many
cases is unimportant (i.e., will have no effect on routing).
Area ID
The OSPF area to which the host belongs.
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RFC 2178 OSPF Version 2 July 1997D. Authentication
All OSPF protocol exchanges are authenticated. The OSPF packet
header (see Section A.3.1) includes an authentication type field, and
64-bits of data for use by the appropriate authentication scheme
(determined by the type field).
The authentication type is configurable on a per-interface (or
equivalently, on a per-network/subnet) basis. Additional
authentication data is also configurable on a per-interface basis.
Authentication types 0, 1 and 2 are defined by this specification.
All other authentication types are reserved for definition by the
IANA (iana@ISI.EDU). The current list of authentication types is
described below in Table 20.
AuType Description
___________________________________________
0 Null authentication
1 Simple password
2 Cryptographic authentication
All others Reserved for assignment by the
IANA (iana@ISI.EDU)
Table 20: OSPF authentication types.
D.1 Null authentication
Use of this authentication type means that routing exchanges over the
network/subnet are not authenticated. The 64-bit authentication field
in the OSPF header can contain anything; it is not examined on packet
reception. When employing Null authentication, the entire contents of
each OSPF packet (other than the 64-bit authentication field) are
checksummed in order to detect data corruption.
D.2 Simple password authentication
Using this authentication type, a 64-bit field is configured on a
per-network basis. All packets sent on a particular network must
have this configured value in their OSPF header 64-bit authentication
field. This essentially serves as a "clear" 64- bit password. In
addition, the entire contents of each OSPF packet (other than the
64-bit authentication field) are checksummed in order to detect data
corruption.
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RFC 2178 OSPF Version 2 July 1997
Simple password authentication guards against routers inadvertently
joining the routing domain; each router must first be configured with
its attached networks' passwords before it can participate in
routing. However, simple password authentication is vulnerable to
passive attacks currently widespread in the Internet (see [Ref16]).
Anyone with physical access to the network can learn the password and
compromise the security of the OSPF routing domain.
D.3 Cryptographic authentication
Using this authentication type, a shared secret key is configured in
all routers attached to a common network/subnet. For each OSPF
protocol packet, the key is used to generate/verify a "message
digest" that is appended to the end of the OSPF packet. The message
digest is a one-way function of the OSPF protocol packet and the
secret key. Since the secret key is never sent over the network in
the clear, protection is provided against passive attacks.
The algorithms used to generate and verify the message digest are
specified implicitly by the secret key. This specification completely
defines the use of OSPF Cryptographic authentication when the MD5
algorithm is used.
In addition, a non-decreasing sequence number is included in each
OSPF protocol packet to protect against replay attacks. This
provides long term protection; however, it is still possible to
replay an OSPF packet until the sequence number changes. To implement
this feature, each neighbor data structure
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Key ID | Auth Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cryptographic sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: Usage of the Authentication field
in the OSPF packet header when Cryptographic
Authentication is employed
contains a new field called the "cryptographic sequence number".
This field is initialized to zero, and is also set to zero whenever
the neighbor's state transitions to "Down". Whenever an OSPF packet
is accepted as authentic, the cryptographic sequence number is set to
the received packet's sequence number.
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This specification does not provide a rollover procedure for the
cryptographic sequence number. When the cryptographic sequence number
that the router is sending hits the maximum value, the router should
reset the cryptographic sequence number that it is sending back to 0.
After this is done, the router's neighbors will reject the router's
OSPF packets for a period of RouterDeadInterval, and then the router
will be forced to reestablish all adjacencies over the interface.
However, it is expected that many implementations will use "seconds
since reboot" (or "seconds since 1960", etc.) as the cryptographic
sequence number. Such a choice will essentially prevent rollover,
since the cryptographic sequence number field is 32 bits in length.
The OSPF Cryptographic authentication option does not provide
confidentiality.
When cryptographic authentication is used, the 64-bit Authentication
field in the standard OSPF packet header is redefined as shown in
Figure 18. The new field definitions are as follows:
Key ID
This field identifies the algorithm and secret key used to create
the message digest appended to the OSPF packet. Key Identifiers
are unique per-interface (or equivalently, per- subnet).
Auth Data Len
The length in bytes of the message digest appended to the OSPF
packet.
Cryptographic sequence number
An unsigned 32-bit non-decreasing sequence number. Used to guard
against replay attacks.
The message digest appended to the OSPF packet is not actually
considered part of the OSPF protocol packet: the message digest is
not included in the OSPF header's packet length, although it is
included in the packet's IP header length field.
Each key is identified by the combination of interface and Key ID. An
interface may have multiple keys active at any one time. This
enables smooth transition from one key to another. Each key has four
time constants associated with it. These time constants can be
expressed in terms of a time-of-day clock, or in terms of a router's
local clock (e.g., number of seconds since last reboot):
KeyStartAccept
The time that the router will start accepting packets that
have been created with the given key.
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RFC 2178 OSPF Version 2 July 1997
KeyStartGenerate
The time that the router will start using the key for packet
generation.
KeyStopGenerate
The time that the router will stop using the key for packet
generation.
KeyStopAccept
The time that the router will stop accepting packets that
have been created with the given key.
In order to achieve smooth key transition, KeyStartAccept should be
less than KeyStartGenerate and KeyStopGenerate should be less than
KeyStopAccept. If KeyStopGenerate and KeyStopAccept are left
unspecified, the key's lifetime is infinite. When a new key replaces
an old, the KeyStartGenerate time for the new key must be less than
or equal to the KeyStopGenerate time of the old key.
Key storage should persist across a system restart, warm or cold, to
avoid operational issues. In the event that the last key associated
with an interface expires, it is unacceptable to revert to an
unauthenticated condition, and not advisable to disrupt routing.
Therefore, the router should send a "last authentication key
expiration" notification to the network manager and treat the key as
having an infinite lifetime until the lifetime is extended, the key
is deleted by network management, or a new key is configured.
D.4 Message generation
After building the contents of an OSPF packet, the authentication
procedure indicated by the sending interface's Autype value is called
before the packet is sent. The authentication procedure modifies the
OSPF packet as follows.
D.4.1 Generating Null authentication
When using Null authentication, the packet is modified as follows:
(1) The Autype field in the standard OSPF header is set to
0.
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RFC 2178 OSPF Version 2 July 1997
(2) The checksum field in the standard OSPF header is set to
the standard IP checksum of the entire contents of the packet,
starting with the OSPF packet header but excluding the 64-bit
authentication field. This checksum is calculated as the 16-bit
one's complement of the one's complement sum of all the 16-bit
words in the packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit words, the
packet is padded with a byte of zero before checksumming.
D.4.2 Generating Simple password authentication
When using Simple password authentication, the packet is modified as
follows:
(1) The Autype field in the standard OSPF header is set to 1.
(2) The checksum field in the standard OSPF header is set to the
standard IP checksum of the entire contents of the packet,
starting with the OSPF packet header but excluding the 64-bit
authentication field. This checksum is calculated as the 16-bit
one's complement of the one's complement sum of all the 16-bit
words in the packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit words, the
packet is padded with a byte of zero before checksumming.
(3) The 64-bit authentication field in the OSPF packet header
is set to the 64-bit password (i.e., authentication key) that has
been configured for the interface.
D.4.3 Generating Cryptographic authentication
When using Cryptographic authentication, there may be multiple keys
configured for the interface. In this case, among the keys that are
valid for message generation (i.e, that have KeyStartGenerate <=
current time < KeyStopGenerate) choose the one with the most recent
KeyStartGenerate time. Using this key, modify the packet as follows:
(1) The Autype field in the standard OSPF header is set to
2.
(2) The checksum field in the standard OSPF header is not
calculated, but is instead set to 0.
(3) The Key ID (see Figure 18) is set to the chosen key's
Key ID.
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RFC 2178 OSPF Version 2 July 1997
(4) The Auth Data Len field is set to the length in bytes of
the message digest that will be appended to the OSPF packet. When
using MD5 as the authentication algorithm, Auth Data Len will be
16.
(5) The 32-bit Cryptographic sequence number (see Figure 18)
is set to a non-decreasing value (i.e., a value at least as large
as the last value sent out the interface). The precise values to
use in the cryptographic sequence number field are
implementation-specific. For example, it may be based on a
simple counter, or be based on the system's clock.
(6) The message digest is then calculated and appended to
the OSPF packet. The authentication algorithm to be used in
calculating the digest is indicated by the key itself. Input to
the authentication algorithm consists of the OSPF packet and the
secret key. When using MD5 as the authentication algorithm, the
message digest calculation proceeds as follows:
(a) The 16 byte MD5 key is appended to the OSPF packet.
(b) Trailing pad and length fields are added, as specified in
[Ref17].
(c) The MD5 authentication algorithm is run over the
concatenation of the OSPF packet, secret key, pad and
length fields, producing a 16 byte message digest (see
[Ref17]).
(d) The MD5 digest is written over the OSPF key (i.e.,
appended to the original OSPF packet). The digest is not
counted in the OSPF packet's length field, but is included
in the packet's IP length field. Any trailing pad or
length fields beyond the digest are not counted or
transmitted.
D.5 Message verification
When an OSPF packet has been received on an interface, it must be
authenticated. The authentication procedure is indicated by the
setting of Autype in the standard OSPF packet header, which matches
the setting of Autype for the receiving OSPF interface.
If an OSPF protocol packet is accepted as authentic, processing of
the packet continues as specified in Section 8.2. Packets which fail
authentication are discarded.
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RFC 2178 OSPF Version 2 July 1997D.5.1 Verifying Null authentication
When using Null authentication, the checksum field in the OSPF header
must be verified. It must be set to the 16-bit one's complement of
the one's complement sum of all the 16-bit words in the packet,
excepting the authentication field. (If the packet's length is not
an integral number of 16-bit words, the packet is padded with a byte
of zero before checksumming.)
D.5.2 Verifying Simple password authentication
When using Simple password authentication, the received OSPF packet
is authenticated as follows:
(1) The checksum field in the OSPF header must be verified.
It must be set to the 16-bit one's complement of the
one's complement sum of all the 16-bit words in the
packet, excepting the authentication field. (If the
packet's length is not an integral number of 16-bit
words, the packet is padded with a byte of zero before
checksumming.)
(2) The 64-bit authentication field in the OSPF packet
header must be equal to the 64-bit password (i.e.,
authentication key) that has been configured for the
interface.
D.5.3 Verifying Cryptographic authentication
When using Cryptographic authentication, the received OSPF packet is
authenticated as follows:
(1) Locate the receiving interface's configured key having
Key ID equal to that specified in the received OSPF
packet (see Figure 18). If the key is not found, or if
the key is not valid for reception (i.e., current time <
KeyStartAccept or current time >= KeyStopAccept), the
OSPF packet is discarded.
(2) If the cryptographic sequence number found in the OSPF
header (see Figure 18) is less than the cryptographic
sequence number recorded in the sending neighbor's data
structure, the OSPF packet is discarded.
(3) Verify the appended message digest in the following
steps:
(a) The received digest is set aside.
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RFC 2178 OSPF Version 2 July 1997
(b) A new digest is calculated, as specified in Step 6
of Section D.4.3.
(c) The calculated and received digests are compared. If
they do not match, the OSPF packet is discarded. If
they do match, the OSPF protocol packet is accepted
as authentic, and the "cryptographic sequence
number" in the neighbor's data structure is set to
the sequence number found in the packet's OSPF
header.
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RFC 2178 OSPF Version 2 July 1997E. An algorithm for assigning Link State IDs
The Link State ID in AS-external-LSAs and summary-LSAs is usually set
to the described network's IP address. However, if necessary one or
more of the network's host bits may be set in the Link State ID.
This allows the router to originate separate LSAs for networks having
the same address, yet different masks. Such networks can occur in the
presence of supernetting and subnet 0s (see [Ref10]).
This appendix gives one possible algorithm for setting the host bits
in Link State IDs. The choice of such an algorithm is a local
decision. Separate routers are free to use different algorithms,
since the only LSAs affected are the ones that the router itself
originates. The only requirement on the algorithms used is that the
network's IP address should be used as the Link State ID whenever
possible; this maximizes interoperability with OSPF implementations
predating RFC 1583.
The algorithm below is stated for AS-external-LSAs. This is only for
clarity; the exact same algorithm can be used for summary-LSAs.
Suppose that the router wishes to originate an AS-external-LSA for a
network having address NA and mask NM1. The following steps are then
used to determine the LSA's Link State ID:
(1) Determine whether the router is already originating an AS-
external-LSA with Link State ID equal to NA (in such an LSA the
router itself will be listed as the LSA's Advertising Router).
If not, the Link State ID is set equal to NA and the algorithm
terminates. Otherwise,
(2) Obtain the network mask from the body of the already existing
AS-external-LSA. Call this mask NM2. There are then two cases:
o NM1 is longer (i.e., more specific) than NM2. In this case,
set the Link State ID in the new LSA to be the network
[NA,NM1] with all the host bits set (i.e., equal to NA or'ed
together with all the bits that are not set in NM1, which is
network [NA,NM1]'s broadcast address).
o NM2 is longer than NM1. In this case, change the existing
LSA (having Link State ID of NA) to reference the new
network [NA,NM1] by incrementing the sequence number,
changing the mask in the body to NM1 and inserting the cost
of the new network. Then originate a new LSA for the old
network [NA,NM2], with Link State ID equal to NA or'ed
together with the bits that are not set in NM2 (i.e.,
network [NA,NM2]'s broadcast address).
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The above algorithm assumes that all masks are contiguous; this
ensures that when two networks have the same address, one mask is
more specific than the other. The algorithm also assumes that no
network exists having an address equal to another network's broadcast
address. Given these two assumptions, the above algorithm always
produces unique Link State IDs. The above algorithm can also be
reworded as follows: When originating an AS-external-LSA, try to use
the network number as the Link State ID. If that produces a
conflict, examine the two networks in conflict. One will be a subset
of the other. For the less specific network, use the network number
as the Link State ID and for the more specific use the network's
broadcast address instead (i.e., flip all the "host" bits to 1). If
the most specific network was originated first, this will cause you
to originate two LSAs at once.
As an example of the algorithm, consider its operation when the
following sequence of events occurs in a single router (Router A).
(1) Router A wants to originate an AS-external-LSA for
[10.0.0.0,255.255.255.0]:
(a) A Link State ID of 10.0.0.0 is used.
(2) Router A then wants to originate an AS-external-LSA for
[10.0.0.0,255.255.0.0]:
(a) The LSA for [10.0.0,0,255.255.255.0] is reoriginated using a
new Link State ID of 10.0.0.255.
(b) A Link State ID of 10.0.0.0 is used for
[10.0.0.0,255.255.0.0].
(3) Router A then wants to originate an AS-external-LSA for
[10.0.0.0,255.0.0.0]:
(a) The LSA for [10.0.0.0,255.255.0.0] is reoriginated using a
new Link State ID of 10.0.255.255.
(b) A Link State ID of 10.0.0.0 is used for
[10.0.0.0,255.0.0.0].
(c) The network [10.0.0.0,255.255.255.0] keeps its Link State ID
of 10.0.0.255.
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RFC 2178 OSPF Version 2 July 1997F. Multiple interfaces to the same network/subnet
There are at least two ways to support multiple physical interfaces
to the same IP subnet. Both methods will interoperate with
implementations of RFC 1583 (and of course this memo). The two
methods are sketched briefly below. An assumption has been made that
each interface has been assigned a separate IP address (otherwise,
support for multiple interfaces is more of a link-level or ARP issue
than an OSPF issue).
Method 1:
Run the entire OSPF functionality over both interfaces, sending and
receiving hellos, flooding, supporting separate interface and
neighbor FSMs for each interface, etc. When doing this all other
routers on the subnet will treat the two interfaces as separate
neighbors, since neighbors are identified (on broadcast and NBMA
networks) by their IP address.
Method 1 has the following disadvantages:
(1) You increase the total number of neighbors and adjacencies.
(2) You lose the bidirectionality test on both interfaces, since
bidirectionality is based on Router ID.
(3) You have to consider both interfaces together during the
Designated Router election, since if you declare both to be
DR simultaneously you can confuse the tie-breaker (which is
Router ID).
Method 2:
Run OSPF over only one interface (call it the primary interface),
but include both the primary and secondary interfaces in your
Router-LSA.
Method 2 has the following disadvantages:
(1) You lose the bidirectionality test on the secondary
interface.
(2) When the primary interface fails, you need to promote the
secondary interface to primary status.
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RFC 2178 OSPF Version 2 July 1997G. Differences from RFC 1583
This section documents the differences between this memo and RFC1583. All differences are backward-compatible. Implementations of
this memo and of RFC 1583 will interoperate.
G.1 Enhancements to OSPF authentication
An additional OSPF authentication type has been added: the
Cryptographic authentication type. This has been defined so that any
arbitrary "Keyed Message Digest" algorithm can be used for packet
authentication. Operation using the MD5 algorithm is completely
specified (see Appendix D).
A number of other changes were also made to OSPF packet
authentication, affecting the following Sections:
o The authentication type is now specified per-interface,
rather than per-area (Sections 6, 9, C.2 and C.3).
o The OSPF packet header checksum is now considered part of
the authentication procedure, and so has been moved out of the
packet send and receive logic (Sections 8.1 and 8.2) and into the
description of authentication types (Appendix D).
o In Appendix D, sections detailing message generation and
message verification have been added.
o For the OSPF Cryptographic authentication type, a discussion
of key management, including the requirement for simultaneous
support of multiple keys, key lifetimes and smooth key
transition, has been added to Appendix D.
G.2 Addition of Point-to-MultiPoint interface
This memo adds an additional method for running OSPF over non-
broadcast networks: the Point-to-Multipoint network. To implement
this addition, the language of RFC 1583 has been altered slightly.
References to "multi-access" networks have been deleted. The term
"non-broadcast networks" is now used to describe networks which can
connect many routers, but which do not natively support
broadcast/multicast (such as a public Frame relay network). Over
non-broadcast networks, there are two options for running OSPF:
modelling them as "NBMA networks" or as "Point-to-MultiPoint
networks". NBMA networks require full mesh connectivity between
routers; when employing NBMA networks in the presence of partial mesh
connectivity, multiple NBMA networks must be configured, as described
in [Ref15]. In contrast, Point-to-Multipoint networks have been
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designed to work simply and naturally when faced with partial mesh
connectivity.
The addition of Point-to-MultiPoint networks has impacted the text in
many places, which are briefly summarized below:
o Section 2 describing the OSPF link-state database has been
split into additional subsections, with one of the subsections
(Section 2.1.1) describing the differing map representations of
the two non-broadcast network options. This subsection also
contrasts the NBMA network and Point- to-MultiPoint network
options, and describes the situations when one is preferable to
the other.
o In contrast to NBMA networks, Point-to-MultiPoint networks
have the following properties. Adjacencies are established
between all neighboring routers (Sections 4, 7.1, 7.5, 9.5 and
10.4). There is no Designated Router or Backup Designated Router
for a Point-to-MultiPoint network (Sections 7.3 and 7.4). No
network-LSA is originated for Point-to-MultiPoint networks
(Sections 12.4.2 and A.4.3). Router Priority is not configured
for Point-to-MultiPoint interfaces, nor for neighbors on Point-
to-MultiPoint networks (Sections C.3 and C.6).
o The Interface FSM for a Point-to-MultiPoint interface is
identical to that used for point-to-point interfaces. Two states
are possible: "Down" and "Point-to-Point" (Section 9.3).
o When originating a router-LSA, and Point-to-MultiPoint
interface is reported as a collection of "point-to-point links"
to all of the interface's adjacent neighbors, together with a
single stub link advertising the interface's IP address with a
cost of 0 (Section 12.4.1.4).
o When flooding out a non-broadcast interface (when either in
NBMA or Point-to-MultiPoint mode) the Link State Update or Link
State Acknowledgment packet must be replicated in order to be
sent to each of the interface's neighbors (see Sections 13.3 and
13.5).
G.3 Support for overlapping area rangesRFC 1583 requires that all networks falling into a given area range
actually belong to a single area. This memo relaxes that restriction.
This is useful in the following example. Suppose that [10.0.0.0,
255.0.0.0] is carved up into subnets. Most of these subnets are
assigned to a single OSPF area (call it Area X), while a few subnets
are assigned to other areas. In order to get this configuration to
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work with RFC 1583, you must not summarize the subnets of Area X with
the single range [10.0.0.0, 255.0.0.0], because then the subnets of
10.0.0.0 belonging to other areas would become unreachable. However,
with this memo you can summarize the subnets in Area X, provided that
the subnets belonging to other areas are not summarized.
Implementation details for this change can be found in Sections 11.1
and 16.2.
G.4 A modification to the flooding algorithm
The OSPF flooding algorithm has been modified as follows. When a Link
State Update Packet is received that contains an LSA instance which
is actually less recent than the the router's current database copy,
the router will now in most cases respond by flooding back its
database copy. This is in contrast to the RFC 1583 behavior, which
was to simply throw the received LSA away.
Detailed description of the change can be found in Step 8 of Section13.
This change improves MaxAge processing. There are times when MaxAge
LSAs stay in a router's database for extended intervals: 1) when they
are stuck in a retransmission queue on a slow link or 2) when a
router is not properly flushing them from its database, due to
software bugs. The prolonged existence of these MaxAge LSAs can
inhibit the flooding of new instances of the LSA. New instances
typically start with LS sequence number equal to
InitialSequenceNumber, and are treated as less recent (and hence were
discarded according to RFC 1583) by routers still holding MaxAge
instances. However, with the above change to flooding, a router
holding a MaxAge instance will flood back the MaxAge instance. When
this flood reaches the LSA's originator, it will then pick the next
highest LS sequence number and reflood, overwriting the MaxAge
instance.
G.5 Introduction of the MinLSArrival constant
OSPF limits the frequency that new instances of any particular LSA
can be accepted during flooding. This is extra protection, just in
case a neighboring router is violating the mandated limit on LSA
(re)originations (namely, one per LSA in any MinLSInterval).
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In RFC 1583, the frequency at which new LSA instances were accepted
was also set equal to once every MinLSInterval seconds. However, in
some circumstances this led to unwanted link state retransmissions,
even when the LSA originator was obeying the MinLSInterval limit on
originations. This was due to either 1) choice of clock granularity
in some OSPF implementations or 2) differing clock speed in
neighboring routers.
To alleviate this problem, the frequency at which new LSA instances
are accepted during flooding has now been increased to once every
MinLSArrival seconds, whose value is set to 1. This change is
reflected in Steps 5a and 5d of Section 13, and in Appendix B.
G.6 Optionally advertising point-to-point links as subnets
When describing a point-to-point interface in its router-LSA, a
router may now advertise a stub link to the point-to-point network's
subnet. This is specified as an alternative to the RFC 1583 behavior,
which is to advertise a stub link to the neighbor's IP address. See
Sections 12.4.1 and 12.4.1.1 for details.
G.7 Advertising same external route from multiple areas
This document fixes routing loops which can occur in RFC 1583 when
the same external destination is advertised by AS boundary routers in
separate areas. There are two manifestations of this problem. The
first, discovered by Dennis Ferguson, occurs when an aggregated
forwarding address is in use. In this case, the desirability of the
forwarding address can change for the worse as a packet crosses an
area aggregation boundary on the way to the forwarding address, which
in turn can cause the preference of AS-external-LSAs to change,
resulting in a routing loop.
The second manifestation was discovered by Richard Woundy. It is
caused by an incomplete application of OSPF's preference of intra-
area routes over inter-area routes: paths to any given
ASBR/forwarding address are selected first based on intra-area
preference, while the comparison between separate ASBRs/forwarding
addresses is driven only by cost, ignoring intra-area preference. His
example is replicated in Figure 19. Both router A3 and router B3 are
originating an AS-external-LSA for 10.0.0.0/8, with the same type 2
metric. Router A1 selects B1 as its next hop towards 10.0.0.0/8,
based on shorter cost to ASBR B3 (via B1->B2->B3). However, the
shorter route to B3 is not available to B1, due to B1's preference
for the (higher cost) intra-area route to B3 through Area A. This
leads B1 to select A1 as its next hop to 10.0.0.0/8, resulting in a
routing loop.
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RFC 2178 OSPF Version 2 July 1997
The following two changes have been made to prevent these routing
loops:
o When originating a type 3 summary-LSA for a configured area
address range, the cost of the summary-LSA is now set to the
maximum cost of the range's component networks (instead of the
previous algorithm which set the cost to the minimum component
cost). This change affects Sections 3.5 and 12.4.3, Figures 7
and 8, and Tables 6 and 13.
o The preference rules for choosing among multiple AS-
external-LSAs have been changed. Where previously cost was the
only determining factor, now the preference is driven first by
type of path (intra-area or inter-area, through non-backbone area
or through backbone) to the ASBR/forwarding address, using cost
only to break ties. This change affects Sections 16.4 and 16.4.1.
After implementing this change, the example in Figure 19 is modified
as follows. Router A1 now chooses A3 as the next
10.0.0.0/8
----------
|
+----+
| XX |
+----+
RIP / \ RIP
--------------------- --------------------
! !
! !
+----+ +----+ 1 +----+......+----+....
| A3 |------| A1 |---------------| B1 |------| B3 | .
+----+ 6 +----+ +----+ 8 +----+ .
1| . / .
OSPF backbone | . / .
+----+ 2 / .
| B2 |------- Area A.
+----+................
Figure 19: Example routing loop when the
same external route is advertised from multiple
areas
hop to 10.0.0.0/8, while B1 chooses B3 as next hop. The reason for
both choices is that ASBRs/forwarding addresses are now chosen based
first on intra-area preference, and then by cost.
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Unfortunately, this change is not backward compatible. While the
change prevents routing loops when all routers run the new preference
rules, it can actually create routing loops when some routers are
running the new preference rules and other routers implement RFC1583. For this reason, a new configuration parameter has been added:
RFC1583Compatibility. Only when RFC1583Compatibility is set to
"disabled" will the new preference rules take effect. See Appendix C
for more details.
G.8 Retransmission of initial Database Description packets
This memo allows retransmission of initial Database Description
packets, without resetting the state of the adjacency. In some
environments, retransmission of the initial Database Description
packet may be unavoidable. For example, the link delay incurred by a
satellite link may exceed the value configured for an interface's
RxmtInterval. In RFC 1583 such an environment prevents a full
adjacency from ever forming.
In this memo, changes have been made in the reception of Database
Description packets so that retransmitted initial Database
Description packets are treated identically to any other
retransmitted Database Description packets. See Section 10.6 for
details.
G.9 Detecting interface MTU mismatches
When two neighboring routers have a different interface MTU for their
common network segment, serious problems can ensue: large packets are
prevented from being successfully transferred from one router to the
other, impairing OSPF's flooding algorithm and possibly creating
"black holes" for user data traffic.
This memo provides a fix for the interface MTU mismatch problem by
advertising the interface MTU in Database Description packets. When a
router receives a Database description packet advertising an MTU
larger than the router can receive, the router drops the Database
Description packet. This prevents an adjacency from forming, telling
OSPF flooding and user data traffic to avoid the connection between
the two routers. For more information, see Sections 10.6, 10.8, and
A.3.3.
G.10 Deleting the TOS routing option
The TOS routing option has been deleted from OSPF. This action was
required by the Internet standards process ([Ref24]), due to lack of
implementation experience with OSPF's TOS routing. However, for
backward compatibility the formats of OSPF's various LSAs remain
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unchanged, maintaining the ability to specify TOS metrics in router-
LSAs, summary-LSAs, ASBR-summary-LSAs, and AS-external-LSAs (see
Sections 12.3, A.4.2, A.4.4, and A.4.5).
To see OSPF's original TOS routing design, consult [Ref9].
Security Considerations
All OSPF protocol exchanges are authenticated. OSPF supports multiple
types of authentication; the type of authentication in use can be
configured on a per network segment basis. One of OSPF's
authentication types, namely the Cryptographic authentication option,
is believed to be secure against passive attacks and provide
significant protection against active attacks. When using the
Cryptographic authentication option, each router appends a "message
digest" to its transmitted OSPF packets. Receivers then use the
shared secret key and received digest to verify that each received
OSPF packet is authentic.
The quality of the security provided by the Cryptographic
authentication option depends completely on the strength of the
message digest algorithm (MD5 is currently the only message digest
algorithm specified), the strength of the key being used, and the
correct implementation of the security mechanism in all communicating
OSPF implementations. It also requires that all parties maintain the
secrecy of the shared secret key.
None of the OSPF authentication types provide confidentiality. Nor do
they protect against traffic analysis. Key management is also not
addressed by this memo.
For more information, see Sections 8.1, 8.2, and Appendix D.
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